Comparison of different SQL implementations

The goal of this page was to gather information relevant for people who are porting SQL from one product to another and/or are interested in possibilities and limits of 'cross-product' SQL.

The following tables compare how different DBMS products handled various SQL (and related) features. If possible, the tables also stated how the implementations should do things, according to the SQL standard.

I'm sorry about the colors. They were a result of wanting to mark each DBMS differently and at the same time wanting to be relatively nice to printers.

Unfortunately, I don't have the time and motivation to keep this page up-to-date any longer.

The following SQL standard and implementations have been examined, if not otherwise stated:

The products are running with their default settings. This is important for MySQL and MSSQL: Their interpretation of SQL may be changed rather drastically by adjusting certain configuration options, potentially increasing the level of standard compliance (for MySQL, there is a dedicated documentation page about this). However, such non-default configuration options are not of great value for people writing SQL applications because the developer often cannot rely on non-default configuration settings.

Standard Views are part of the standard, and they may be updated, as long as it 'makes sense'. SQL:2008 has a rather complicated set of rules governing when a view is updatable, basically saying that a view is updatable, as long as the update-operation translates into an unambiguous change. SQL-92 was more restrictive, specifying that updatable views cannot be derived from more than one base table. PostgreSQL Has views. Breaks that standard by not allowing updates to views; offers the non-standard 'rules'-system as a work-around. DB2 Conforms to at least SQL-92. MSSQL Conforms to at least SQL-92. MySQL Conforms to at least SQL-92. Oracle Conforms to at least SQL-92. Informix Conforms to at least SQL-92.

All the DBMSes support basic INNER JOINs, but vary in their support for other join types.

In the following feature chart, a means yes; an empty table cell means no.

Join type/feature PostgreSQL DB2 MSSQL MySQL Oracle Informix Natural joins (only tested: NATURAL LEFT JOIN ) USING -clause FULL joins1 (tested: SELECT...FULL JOIN...ON...=... ) Explicit CROSS JOIN (cartesian product)

Remarks:

Note that FULL joins may be emulated with a union of a left and a right join.

Objective: An existing table, t1 needs to be copied to a new table, t2, without copying data. I.e., only the structure/definition of the table is copied.

Standard Optional feature T171 defines LIKE clause in table definition:

CREATE TABLE t2 ( LIKE t1 ) The DBMS may support an extension of this (feature T173) which allows for more table properties to be copied:

CREATE TABLE t2 ( LIKE t1 INCLUDING IDENTITY INCLUDING DEFAULTS INCLUDING GENERATED ) If INCLUDING DEFAULTS is not specified, column defaults will not be part of t2; likewise with IDENTITY and GENERATED properties. Triggers, CHECK constraints, and other 'non-trivial' table features are not copied to the new table. PostgreSQL Complies with the core of the feature (T171). The extended T173 feature is only partially supported, and extended with a few non-standard options: The INCLUDING IDENTITY and INCLUDING GENERATED options are not supported

and options are not supported INCLUDING CONSTRAINTS and INCLUDING INDEXES options are added PostgreSQL does not allow you to copy the structure of a view, using CREATE TABLE ... (LIKE ...) . For that, you may use another construct:

CREATE TABLE copytable AS SELECT * FROM viewname WHERE false Documentation DB2 Behaves as if inspired by the standard. I.e., DB2 conforms to the standard, except: the LIKE ... clause is stated outside any parenthesis

clause is stated outside any parenthesis the extended INCLUDING GENERATED option is not supported

option is not supported DB2 defaults to copy IDENTITY, DEFAULTS, and GENERATED properties, unless EXCLUDING IDENTITY and/or EXCLUDING DEFAULTS is specified. Example:

CREATE TABLE t2 LIKE t1 INCLUDING DEFAULTS DB2 allows you to copy the structure of a view into a table. Documentation MSSQL Does not support the standard. Instead, MSSQL has a special SELECT ... INTO ... FROM ... construct which can be combined with an impossible WHERE-clause to copy structure only:

SELECT * INTO t2 FROM t1 WHERE 1<>1 The source (t1) may be a view, as well as a table. SELECT ... INTO copies NOT NULL column attributes, but nothing else. Documentation MySQL Complies with the core of the feature (T171), but not with the extended features (T173). MySQL does not allow you to copy the structure of a view into a table. Documentation Oracle Does not support the standard. Oracle lets you copy a table structure using a special CREATE TABLE ... AS construct, combined with an impossible WHERE -clause: CREATE TABLE t2 AS SELECT * FROM t1 WHERE 1<>1 Documentation Informix On my TODO.

Standard The SQL-standard states that relations are unordered, but result sets may be ordered when returned to the user through a cursor: DECLARE cursorname CURSOR FOR

SELECT ... FROM ... WHERE ...

ORDER BY column_name1,column_name2,... The DBMS may additionally allow ORDER BY outside cursor definitions (optional feature IDs F850, F851, F852, F855).(Since SQL:2008) The standard doesn't specify how NULLs should be ordered in comparison with non-NULL values, except that any two NULLs are to be considered equally ordered, and that NULLs should sort either above or below all non-NULL values. However, the DBMS may optionally (as part of feature ID T611, "Elementary OLAP operations") allow the user to specify whether NULLs should sort first or last:

... ORDER BY ... NULLS FIRST

or

... ORDER BY ... NULLS LAST PostgreSQL As well as in cursor definitions, it allows ORDER BY in other contexts. By default, NULLs are considered higher than any non-NULL value; however,(since version 8.3) this sorting behaviour may be changed by adding NULLS FIRST or NULLS LAST to the ORDER BY expression. DOCUMENTATION DB2 As well as in cursor definitions, it allows ORDER BY in other contexts. NULLs are considered higher than any non-NULL value. DOCUMENTATION MSSQL As well as in cursor definitions, it allows ORDER BY in other contexts. NULLs are considered lower than any non-NULL value. DOCUMENTATION MySQL As well as in cursor definitions, it allows ORDER BY in other contexts. NULLs are considered lower than any non-NULL value, except if a - (minus) character is added before the column name and ASC is changed to DESC, or DESC to ASC; this minus-before-column-name feature seems undocumented. Documentation Oracle As well as in cursor definitions, it allows ORDER BY in other contexts. By default, NULLs are considered higher than any non-NULL value; however, this sorting behaviour may be changed by adding NULLS FIRST or NULLS LAST to the ORDER BY expression. Beware of Oracle's strange treatment of empty strings and NULLs as the same 'value'. DOCUMENTATION Informix As well as in cursor definitions, it allows ORDER BY in other contexts. NULLs are considered lower than any non-NULL value. DOCUMENTATION

Objective: Want to only get n rows in the result set. Usually only makes sense in connection with an ORDER BY expression.

Note: This is not the same as a top-n query — see next section.

Note also: Some of the queries below may not be legal in all situations, such as in views or sub-queries.

By the way, Use The Index, Luke! has a page about this subject.

Standard The SQL standard provides three ways of performing a 'simple limit': Using FETCH FIRST : (since SQL:2008) Non-core feature IDs F856, F857, F858, and F859 describe using

SELECT ... FROM ... WHERE ... ORDER BY ... FETCH FIRST n ROWS ONLY You may write ROW instead of ROW S .

: Using a Window function : (since SQL:2003) Non-core Feature ID T611 specifies window functions, of which one is ROW_NUMBER() OVER :



SELECT * FROM (

SELECT

ROW_NUMBER() OVER (ORDER BY key ASC) AS rownumber,

columns

FROM tablename

) AS foo

WHERE rownumber <= n

: Using a cursor : If your application is stateful (in contrast to web applications which normally have to be seen as stateless), then you might look at cursors (core feature ID E121) instead. This involves: DECLARE cursor-name CURSOR FOR ... OPEN cursor-name FETCH ... CLOSE cursor-name

: PostgreSQL Supports all standards-based approaches. In old PostgreSQL versions (versions 8.3 and older), a special PostgreSQL (and MySQL) specific method was used: SELECT columns

FROM tablename

ORDER BY key ASC

LIMIT n Note that LIMIT changes the semantics of SELECT...FOR UPDATE . Documentation: FETCH FIRST/LIMIT

Window functions DB2 Supports all standards-based approaches. Documentation: OLAP functions

FETCH FIRST (general page about the SELECT statement; use your browser's search function to locate FETCH FIRST ) MSSQL Supports the ROW_NUMBER() (since MSSQL 2005) and cursor standards-based approaches; doesn't support FETCH FIRST . MSSQL 2000 didn't support ROW_NUMBER() . Instead, a MSSQL 2000-specific syntax was needed:

SELECT TOP n columns

FROM tablename

ORDER BY key ASC

The TOP construct is still available in MSSQL 2008, and it's handy for casual SQL work. Documentation MySQL Doesn't support the standard. Alternative solution: SELECT columns

FROM tablename

ORDER BY key ASC

LIMIT n Documentation Oracle Supports ROW_NUMBER ; doesn't support FETCH FIRST . As Oracle doesn't allow AS for subquery naming (and doesn't need a subquery-name at all in this case), the standard SQL code above needs to be rewritten slightly: SELECT * FROM (

SELECT

ROW_NUMBER() OVER (ORDER BY key ASC) AS rownumber,

columns

FROM tablename

)

WHERE rownumber <= n Documentation A reader of this page told me that using the Oracle-specific ROWNUM 'magic' column yields better performance than using the ROW_NUMBER function. You may want to experiment with this. Ask Tom has an article on ROWNUM. Informix Supports neither ROW_NUMBER(), nor FETCH FIRST. Alternative solution (which is illegal in plain sub-queries):

SELECT FIRST n columns

FROM tablename

ORDER BY key ASC Documentation

Objective: Like the simple limit-query above, but include rows with tie conditions. Thus, the query may return more than n rows.

Some call this a quota-query.

The following examples are based on this table:

SELECT * FROM person ORDER BY age ASC; +----------+-------------+-----+ |PERSON_ID | PERSON_NAME | AGE | +----------+-------------+-----+ | 7 | Hilda | 12 | | 8 | Bill | 12 | | 4 | Joe | 23 | | 2 | Veronica | 23 | | 3 | Michael | 27 | | 9 | Marianne | 27 | | 1 | Ben | 50 | | 10 | Michelle | 50 | | 5 | Irene | 77 | | 6 | Vivian | 77 | +----------+-------------+-----+

Now, we only want the three (n=3) youngest persons displayed, i.e. a result set like this:

+----------+-------------+-----+ |PERSON_ID | PERSON_NAME | AGE | +----------+-------------+-----+ | 7 | Hilda | 12 | | 8 | Bill | 12 | | 4 | Joe | 23 | | 2 | Veronica | 23 | +----------+-------------+-----+

Objective: Want to only get n rows in the result set, and we want the first skip rows in the result set discarded. Usually only makes sense in connection with an ORDER BY expression.

In the recipes below, basic ordering is ASCending, i.e. lowest-first queries. If you want the opposite, then change ASC->DESC and DESC->ASC at the places emphasized like this.

By the way, Use the Index, Luke! has a page about this.

Standard The SQL standard provides three ways of performing 'limit with offset': Using OFFSET and FETCH FIRST : (since SQL:2008) SELECT...

FROM ...

WHERE ...

ORDER BY ...

OFFSET skip ROWS

FETCH FIRST n ROWS ONLY You may write ROW instead of ROW S .

and : Using a window function : (since SQL:2003) Non-core Feature ID T611 specifies window functions, one of which is ROW_NUMBER() OVER :



SELECT * FROM (

SELECT

ROW_NUMBER() OVER (ORDER BY key ASC ) AS rownum,

columns

FROM tablename

) AS foo

WHERE rownum > skip AND rownum <= (n+skip)

: Using a cursor : You may use a cursor (core feature ID E121), if the programming environment permits it. This involves: DECLARE cursor-name CURSOR FOR ... OPEN cursor-name FETCH RELATIVE number-of-rows-to-skip ... CLOSE cursor-name

: PostgreSQL Supports all the standards-based approaches. In version 8.3 and older, cursors should be used, or a special construct:

SELECT columns

FROM tablename

ORDER BY key ASC

LIMIT n OFFSET skip Documentation: OFFSET...FETCH/LIMIT...OFFSET

Window functions DB2 Supports the window function based approach. Regarding cursors: DB2 for Linux/Unix/Windows doesn't support FETCH RELATIVE (which is strange, because DB2 for the mainframe seems to support it). Instead, see if the DB2 driver for your programming environment supports SQLFetchScroll() . Documentation: OLAP functions, the FETCH statement. MSSQL Supports the window function and cursor based approaches. MSSQL 2000 didn't support ROW_NUMBER() ; instead, a MSSQL-specific syntax had to be used:

SELECT * FROM (

SELECT TOP n * FROM (

SELECT TOP z columns -- (z=n+skip)

FROM tablename

ORDER BY key ASC

) AS FOO ORDER BY key DESC -- ('FOO' may be anything)

) AS BAR ORDER BY key ASC -- ('BAR' may be anything)

Documentation MySQL Doesn't support the standard approaches. Alternative solution:



SELECT columns

FROM tablename

ORDER BY key ASC

LIMIT n OFFSET skip In older versions of MySQL, the LIMIT-syntax is less clear:

... LIMIT [skip,] n

(i.e. the skip argument is optional).

The old syntax is still supported by later MySQL versions (the old syntax is widely used). Documentation Oracle Supports ROW_NUMBER() . I'm unsure if Oracle's cursor support is standards-compliant. As Oracle doesn't accept AS for subquery naming (and doesn't require naming of subqueries in this case), the standard SQL solution has to be re-written slightly. An other reason for the re-write is that ROWNUM is a reserved word in Oracle, with special meaning. The Oracle code becomes: SELECT * FROM (

SELECT

ROW_NUMBER() OVER (ORDER BY key ASC) AS rn,

columns

FROM tablename

)

WHERE rn > skip AND rn <= (n+skip) Documentation A reader of this page told me that using the Oracle-specific ROWNUM 'magic' column yields better performance than using the ROW_NUMBER function. You may want to experiment with this. Ask Tom has an article on ROWNUM. Informix Supports neither OFFSET ... FETCH FIRST nor ROW_NUMBER . Supports cursors. An alternative to using cursors is to us an Informix-specific construct:

SELECT SKIP skip FIRST n *

FROM tablename Documentation: SKIP and FIRST

Note: FETCH FIRST/LIMIT/TOP queries with offset are often used in a result presentation context: To retrieve only—say—30 rows at a time so that the end-user isn't overwhelmed by the complete result set, but instead is offered a paginated result presentation. In this case, be careful not to (only) sort on a non-unique column. Consider the following example (where PostgreSQL is used): SELECT * FROM person ORDER BY age ASC; person_id | person_name | age -----------+-------------+----- 7 | Hilda | 12 8 | Bill | 12 4 | Joe | 23 2 | Veronica | 23 3 | Michael | 27 9 | Marianne | 27 1 | Ben | 50 10 | Michelle | 50 5 | Irene | 77 6 | Vivian | 77 When ordering is performed on the non-unique age-value, ties may occur and it's not guaranteed that the DBMS will fetch the rows in the same order every time. Instead of the above listing, the DBMS is allowed to return the following display order where Michael and Marianne are displayed in the opposite order compared to above: SELECT * FROM person ORDER BY age ASC; person_id | person_name | age -----------+-------------+----- 7 | Hilda | 12 8 | Bill | 12 4 | Joe | 23 2 | Veronica | 23 9 | Marianne | 27 3 | Michael | 27 1 | Ben | 50 10 | Michelle | 50 5 | Irene | 77 6 | Vivian | 77 Now, suppose the end-user wants the results displayed five rows at a time. The result set is fetched in two queries where the DBMS happens to sort differently, as above. We will use PostgreSQL's legacy syntax in the example: SELECT * FROM person ORDER BY age ASC LIMIT 5; person_id | person_name | age -----------+-------------+----- 7 | Hilda | 12 8 | Bill | 12 4 | Joe | 23 2 | Veronica | 23 3 | Michael | 27 SELECT * FROM person ORDER BY age ASC LIMIT 5 OFFSET 5; person_id | person_name | age -----------+-------------+----- 3 | Michael | 27 1 | Ben | 50 10 | Michelle | 50 5 | Irene | 77 6 | Vivian | 77 Notice that Marianne was not displayed in any of the two split result set presentations. The problem could be avoided if the result set ordering had been done in a deterministic way, i.e. where the unique person_id value was considered in case of a tie:

SELECT * FROM person ORDER BY age ASC, person_id ASC ...

This is safer than to pray for the DBMS to behave in a predictable way when handling non-unique values. Note: If the table is updated between parts of the result set pagination, then the user might still get an inconsistent presentation. If you want to guard against this, too, then you should see if use of an insensitive cursor is an option in your application. Use of cursors to paginate result sets usually require that your application is stateful, which is not the case in many web-application settings. Alternatively, you could let the application cache the complete result set (e.g. in a session if your web application environment provides for sessions).

Standard An optional SQL feature is row value constructors (feature ID F641). One handy use of row value constructors is when inserting several rows at a time, such as: INSERT INTO tablename

VALUES (0,'foo') , (1,'bar') , (2,'baz'); — which may be read as a shorthand for INSERT INTO tablename VALUES (0,'foo');

INSERT INTO tablename VALUES (1,'bar');

INSERT INTO tablename VALUES (2,'baz'); PostgreSQL Supported.(since version 8.2) DB2 Supported. MSSQL Supported.(since version 2008) MySQL Supported. Oracle An Oracle-specific kludge: INSERT INTO tablename

SELECT 0,'foo' FROM DUAL

UNION ALL

SELECT 1,'bar' FROM DUAL

UNION ALL

SELECT 2,'baz' FROM DUAL Alternatively: INSERT ALL

INTO tablename VALUES(0,'foo')

INTO tablename VALUES(1,'bar')

INTO tablename VALUES(2,'baz')

SELECT null FROM dual Informix On my TODO.

Standard The BOOLEAN type is optional (has feature ID T031), which is a bit surprising for such a basic type. However, it seems that endless discussions of how NULL is to be interpreted for a boolean value is holding BOOLEAN from becoming a core type. The standard says that a BOOLEAN may be one of the following literals: TRUE

FALSE

UNKNOWN or NULL (unless prohibited by a NOT NULL constraint) The DBMS may interpret NULL as equivalent to UNKNOWN. It is unclear from the specification if the DBMS must support UNKNOWN, NULL or both as boolean literals. In this author's opinion, you should forget about the UNKNOWN literal in order to simplify the situation and let the normal SQL three-way logic apply. It's defined that TRUE > FALSE (true larger than false). PostgreSQL Follows the standard. Accepts NULL as a boolean literal; doesn't accept UNKNOWN as a boolean literal. Documentation DB2 Doesn't support the BOOLEAN type.



Judging from various JDBC-documentation, it seems that IBM recommends a CHAR(1) field constrained to values '0' and '1' (and perhaps NULL) as the way to store boolean values. MSSQL Doesn't support the BOOLEAN type. Possible alternative type: the BIT type which may have 0 or 1 (or NULL) as value. If you insert an integer value other than these into a field of type BIT, then the inserted value will silently be converted to 1. Rudy Limeback has some notes about oddities with the MSSQL BIT type. Documentation MySQL Offers a non-conforming BOOLEAN type. MySQL's BOOLEAN is one of many aliases to its TINYINT(1) type. (Take care if you use TINYINT(1) and JDBC with MySQL and expect to get non-boolean values from it.) MySQL accepts the literals TRUE and FALSE as aliases to 1 and 0, respectively. However, you may also assign a value of — e.g. — 9 to a column of type BOOLEAN (which is non-conforming). If you use JDBC with MySQL, then BOOLEAN is the preferred type for booleans: MySQL's JDBC-driver implicitly converts between Java's boolean and MySQL's pseudo-BOOLEAN type. Side note: MySQL has a BIT type which may be interesting for people with enormous amounts of boolean-type data. Documentation Oracle Doesn't support the BOOLEAN type.



Judging from various JDBC documentation and a discussion at Ask Tom, it seems that Oracle recommends NUMBER(1) as the way to store boolean values; it's probably wise to constrain such columns to values 0 and 1 (and perhaps NULL). Informix On my TODO.

Warning to JDBC users:

According to the JDBC standard, getBoolean() must convert a SQL-'value' of NULL to the false Java value. To check if the database-value was really NULL, use wasNull().

For the following section, I have used this test-SQL to try to illuminate differences (unfortunately, even standard SQL as simple as this has to be adjusted for some products):

Test steps:

CREATE TABLE chartest (

charval1 CHAR(10) NOT NULL,

charval2 CHAR(10) NOT NULL,

varcharval VARCHAR(30) NOT NULL

);

INSERT INTO chartest VALUES ('aaa','aaa','aaa');

INSERT INTO chartest

VALUES ('aaaaaa ','aaa','aaa'); -- should truncate to 'aaaaaa '

INSERT INTO chartest

VALUES ('aaaaaaaaaaaa','aaa','aaa'); -- should raise error

SELECT * FROM chartest; -- should show two rows

DELETE FROM chartest WHERE charval1='aaaaaa';

SELECT * FROM chartest; -- should show one row

SELECT * FROM chartest WHERE charval1=varcharval;

SELECT charval1 || 'X' AS res FROM chartest;

SELECT CHAR_LENGTH(charval1 || charval2) AS res FROM chartest;

SELECT CHAR_LENGTH(charval1) + CHAR_LENGTH(charval2)

AS res

FROM chartest;

Expected results, after CREATE and INSERTs:

SELECT * FROM chartest; -- should show two rows CHARVAL1 CHARVAL2 VARCHARVAL ========== ========== ============================== aaa aaa aaa aaaaaa aaa aaa DELETE FROM chartest WHERE charval1='aaaaaa'; SELECT * FROM chartest; -- should show one row CHARVAL1 CHARVAL2 VARCHARVAL ========== ========== ============================== aaa aaa aaa SELECT * FROM chartest WHERE charval1=varcharval; CHARVAL1 CHARVAL2 VARCHARVAL ========== ========== ============================== aaa aaa aaa SELECT charval1 || 'X' FROM chartest AS res; res =========== aaa X SELECT CHAR_LENGTH(charval1 || charval2) AS res FROM chartest; res =========== 20 SELECT character_length(charval1) + character_length(charval2) AS res FROM chartest; res ============ 20

Actual results.

Standard Return with an exception state if the inserted string is too long, unless the characters exceeding the limit are all spaces.

Pad CHAR columns with spaces if the inserted string is shorter than the specified CHAR-length.

Pad with trailing spaces as needed when casting or comparing to other string-like values (e.g. VARCHARs). PostgreSQL Stores CHARs in space padded form, but violates the standard by (conceptually) truncating trailing white-space before performing most functions, operators, and comparisons (like the CHARACTER_LENGTH -function and the concatenation( || ) operator). Documentation DB2 Follows the standard. Documentation MSSQL Generally follows standard, but (conceptually) truncates trailing white-space before performing some functions (at least before LEN() ). Documentation MySQL Breaks the standard by silently inserting the string, truncated to specified column CHAR-length.

(It's actually not completely silent, as it issues warnings if values were truncated: If you manually check for warnings, you will know that something bad happened, but not which of the rows are now invalid.)



Violates the standard by effectively truncating all trailing spaces.

The documentation states that MySQL truncates trailing spaces when CHAR values are retrieved. That may be true, but it seems that truncation even happens before the CHAR values are used as input in functions like CONCAT , CHAR_LENGTH , etc. Documentation Oracle Follows the standard, with a minor exception: Oracle doesn't remove trailing spaces which exceed the specified CHAR length, but raises an exception. Documentation Informix On my TODO.

Part of the Core requirements, feature ID F051-03.

Stores year, month, day, hour, minute, second (with fractional seconds; default is 6 fractional digits).

Extension to Core SQL (feature ID F411): TIMESTAMP WITH TIME ZONE which also stores the time zone. Examples of TIMESTAMP literals: TIMESTAMP '2003-07-29 13:19:30'

TIMESTAMP '2003-07-29 13:19:30.5' Examples of TIMESTAMP WITH TIME ZONE literals: TIMESTAMP '2003-07-29 13:19:30+02:00'

TIMESTAMP '2003-07-29 13:19:30.5+02:00' It's strange that TIMESTAMP WITH TIME ZONE literals are not represented as, e.g., TIMESTAMP WITH TIME ZONE '2003-07-29 13:19:30+01:00' , but according to Melton & Simon's book, they aren't. Follows that standard with one exception:

In some cases, TIMESTAMP '2003-08-23 01:02:03 +02:00' is interpreted as a TIMESTAMP WITHOUT TIME ZONE (discarding the ' +02:00 ' part)—not as a TIMESTAMP WITH TIME ZONE value. The standard may be illogical regarding this, but a standard is a standard... Performs good sanity checks on inserted timestamp values; e.g. this will work:

INSERT INTO tablename (columnname)

VALUES (TIMESTAMP '2003-02-28 00:05:00')

while this will fail:

INSERT INTO tablename (columnname)

VALUES (TIMESTAMP '2003-02-29 00:05:00') Documentation DB2 has the TIMESTAMP data type, but not the extended TIMESTAMP WITH TIME ZONE type. Performs good sanity checks on inserted timestamp values; e.g. this will work:

INSERT INTO tablename (columnname)

VALUES ('2003-02-28 00:05:00')

while this will fail:

INSERT INTO tablename (columnname)

VALUES ('2003-02-29 00:05:00') Documentation Note that MSSQL's choice of words related to date and time is confusing: In MSSQL's vocabulary, datetime is a concrete data type, whereas in the SQL standard, datetime is a general term covering the DATE, TIME and TIMESTAMP types. MSSQL has a strange pseudo-type called TIMESTAMP, but has deprecated it; don't use it in new code. The closest match to the SQL standard's TIMESTAMP type is DATETIME. This type stores the combination of date and time. It has a maximum of three fractional digits for seconds. Performs good sanity checks on inserted timestamp values; e.g. this will work:

INSERT INTO tablename (columnname)

VALUES ('2003-02-28 00:05:00')

while this will fail:

INSERT INTO tablename (columnname)

VALUES ('2003-02-29 00:05:00') Documentation No matter what date/time data type chosen in MySQL, storage of fractional seconds and time zones are not supported (the TIME type accepts time literals with fractional seconds, but discards the fractional part when storing the value). You will have to invent your own systems for such information.

Note also, that MySQL's choice of words related to date and time is confusing: In MySQL's vocabulary, datetime is a concrete data type, whereas in the SQL standard, datetime is a general term covering the DATE, TIME and TIMESTAMP types. MySQL has a type called TIMESTAMP, but it is quite different from the standard TIMESTAMP: It's a 'magic' data type with side effects in that it's automatically updated to the current date and time if some criteria are fulfilled. MySQL has a type called DATETIME. Like MySQL's TIMESTAMP type, it stores a combination of date and time without fractional seconds. There are no side effects associated with the DATETIME type—which makes it the closest match to the SQL standard's TIMESTAMP type. By default, MySQL's sanity checks with regard to dates and time are (deliberately) poor. For example, MySQL accepts DATETIME values of '2003-02-29 00:05:00' and '2003-01-32 00:00:00'. Such values yield warnings (which you must check for if you want to be warned), but result in a value of zero being stored. Documentation Follows the standard. Oracle has both the TIMESTAMP and the extended TIMESTAMP WITH TIME ZONE types. A special gotcha applies, though: Oracle forbids columns of type TIMESTAMP WITH TIME ZONE as part of a unique key; this includes primary and foreign keys. Timestamps without time zone (and Oracle's special TIMESTAMP WITH LOCAL TIME ZONE) are accepted. Performs good sanity checks on inserted timestamp values; e.g. this will work:

INSERT INTO tablename (columnname)

VALUES (TIMESTAMP'2003-02-28 00:05:00')

while this will fail:

INSERT INTO tablename (columnname)

VALUES (TIMESTAMP'2003-02-29 00:05:00') Documentation On my TODO.

Standard CHARACTER_LENGTH( argument )

If the optional feature T061 is implemented, the function may be augmented with an indication of string unit:

CHARACTER_LENGTH( argument USING string-unit )

string-unit may be UTF8 , UTF16 , UTF32 . Returns NUMERIC. Returns NULL if the input is NULL.

Alias: CHAR_LENGTH.

The argument may be of type CHAR or VARCHAR.

Part of the Core SQL requirements (feature ID E021-04).

Related function: OCTET_LENGTH. PostgreSQL Follows the standard, providing CHARACTER_LENGTH (and CHAR_LENGTH ). Note that PostgreSQL removes trailing (not leading) space from from CHAR values before counting. Note also that the behaviour of CHARACTER_LENGTH with regard to CHAR values has changed between versions 7.4 and 8.0 of PostgreSQL. Documentation DB2 Has a CHARACTER_LENGTH function, but it's non-compliant because it requires indication of string unit, and db2's string units are different from the standard's. Provides the LENGTH function for those who don't want to think about string units, but the LENGTH function may return wrong results in UTF-8 databases. Note that CHAR values are space-padded (like the standard says they should be), so the length of 'HEY ' is 5. Consider using LENGTH(TRIM(foo)) if you want the length without trailing spaces. Documentation: CHARACTER_LENGTH and LENGTH MSSQL Doesn't have CHARACTER_LENGTH. Provides the LEN and DATALENGTH functions instead (the latter is especially valid for 'special' data types like the TEXT type).

Note that MSSQL's LEN -function removes trailing (not leading) spaces from CHAR values before counting; MSSQL's DATALENGTH doesn't discard spaces. Documentation: LEN and DATALENGTH MySQL Provides CHARACTER_LENGTH.

Aliases: CHAR_LENGTH, LENGTH.

Note that MySQL removes trailing (not leading) spaces from CHAR values before counting. Documentation Oracle Doesn't have CHARACTER_LENGTH. Provides the LENGTH function instead. Behaves in strange ways if the input is the empty string or NULL, because of Oracles non-standard NULL handling (it considers NULL and the empty string identical 'values'). Note that CHAR values are space-padded (like the standard says they should be), so the length of 'HEY ' is 5. Consider using LENGTH(TRIM(TRAILING FROM foo)) if you want the length without leading/trailing spaces. Documentation Informix On my TODO.

Standard The standard defines two variants of the SUBSTRING function: To comply with Core SQL (Feature E021-06), the DBMS must support an 'ordinary' SUBSTRING function which extracts characters from a string:

SUBSTRING( input FROM start-position [FOR length ])

Strings start at position 1. The start-position argument is a numeric value, as is the optional length -argument. If no length parameter is indicated, length becomes infinite (The standard specifies an extra optional argument— USING x —that has to do with Universal Character Sets, e.g. Unicode. x may be one of OCTETS or CHARACTERS.) The result is NULL if any of the arguments is NULL. Some cases of out-of-range values for start-position and length are allowed. Examples: SUBSTRING('12345' FROM 6) yields the empty string.

yields the empty string. A start-position less than 1 effectively sets start-position to 1 and reduces the value of length by 1+abs( start-position ).

I.e., if start-position is -3 and length is 6, then the length value becomes 2.



Another way to put it is that when start-position is negative, a bunch of arbitrary/blank characters are prepended to the input-value. bunch=1- start-position . For an exact definition: see item three in the "General Rules" part of section 6.29 in the standard. The DBMS may optionally offer a regular expression variant (Feature T581) of SUBSTRING:

SUBSTRING( input SIMILAR pattern ESCAPE escape-char )

Pattern deserves some explanation. It's a string which needs to consist of three parts: A part matching before the wanted sub-string, the wanted substring, and a part matching after the wanted substring.

The parts must be separated by a combination of the indicated escape-char (escape-character) and a double-quote ("). Example:

SUBSTRING('abc' SIMILAR 'a#"b#"c' ESCAPE '#')

should yield

b

The pattern description rules in SQL don't completely resemble POSIX regular expressions, as far as I can see. PostgreSQL PostgreSQL provides three SUBSTRING flavors: Ordinary SUBSTRING: As the standard's ordinary SUBSTRING variant.

POSIX regular expression SUBSTRING: Syntax is

SUBSTRING( input FROM pattern-string )

Pattern rules are of the POSIX variant. Returns NULL when pattern doesn't match.

Pattern rules are of the POSIX variant. Returns NULL when pattern doesn't match. Sort-of SQL-style regular expression SUBSTRING: Syntax is

SUBSTRING( input FROM pattern-string FOR escape-char )

Pattern-rules are supposed to match the SQL-standard's rules, although my tests sometimes suggest otherwise (hasn't been reported as bugs, because I'm not completely sure how SQL's regex-rules are supposed to be expressed). Returns NULL when pattern doesn't match. Documentation DB2 Provides (since version 9) the SUBSTRING function, but requires you to indicate string unit by appending " USING unit ".

The unit identifier may be CODEUNITS16 , CODEUNITS32 , or OCTETS . CODEUNITS16 / CODEUNITS32 seem non-standard. The standard's CHARACTERS unit isn't supported by DB2.

Example:

SELECT SUBSTRING( somecolumn FROM 3 USING OCTETS) FROM sometable

SELECT SUBSTRING( somecolumn FROM 3 FOR 2 USING OCTETS) FROM sometable For old DB2 versions, use the non-standard SUBSTR function. DB2 doesn't provide any built-in regular expression facilities at all (but you may manually add PCRE capabilities). Documentation: SUBSTRING and SUBSTR MSSQL MSSQL has a SUBSTRING function, but its syntax differs from that of the standard. The syntax is: SUBSTRING(input, start, length) where start is an integer specifying the beginning of the string, and length is a non-negative integer indicating how many characters to return. MSSQL has no regular expression functionality. Documentation MySQL MySQL supports the standard's ordinary SUBSTRING function, with some twists (see below). No regular expression based substring extraction is supported.



MySQL breaks the standard when negative values are used as either start-position or length: According to the standard, SUBSTRING('abc' FROM -2 FOR 4) should yield 'a' ;in MySQL, the result is 'bc'.

should yield ;in MySQL, the result is 'bc'. According to the standard, SUBSTRING('abc' FROM 2 FOR -4) should yield an error; MySQL returns an empty string. Documentation Oracle Doesn't provide the standard SUBSTRING function. Provides SUBSTR( input , start-pos [, length ]) instead (i.e. length is optional).

Oracle provides a number of SUBSTR-variants (SUBSTRB, SUBSTRC, SUBSTR2, SUBSTR4, same syntax as for SUBSTR), mainly for handling various kinds of non-latin-only string-types.

Oracle doesn't have support for string-extraction with the special SQL-style regular expressions. Instead, it has the REGEXP_SUBSTR function which offers string extraction, using POSIX-style regular expression pattern matching. Documentation: SUBSTR and REGEXP_SUBSTR. Informix On my TODO.

Note: If you find yourself using SUBSTRING in a WHERE-expression, then consider if LIKE could be used instead: The use of LIKE will typically make your DBMS try to use an index, whereas it will typically not try to do so in connection with functions.

REPLACE means a string-function which searches a source string (haystack) for occurrences of a string to be replaced (needle) and replaces it with a new string (replacement).

Standard Not mentioned. May be obtained through a combination of other functions (have a look at the OVERLAY, POSITION and CHARACTER_LENGTH functions).



A de facto standard seems to have emerged with regard to REPLACE:



REPLACE (haystack:string,needle:string,replacement:string)



which means 'replace needle with replacement in the string haystack'. Replacement is done case-sensitively unless otherwise stated.



The REPLACE function may be handy for correcting spelling errors (and other situations):

UPDATE tablename

SET fullname=REPLACE(fullname,'Jeo ','Joe ') PostgreSQL Follows de facto standard.

Documentation DB2 Follows de facto standard.

Documentation MSSQL Follows de facto standard with the exception that MSSQL by default works case insensitively.

Documentation MySQL Follows de facto standard.

MySQL even works case sensitively.1

Note that the REPLACE -function is different from MySQL's non-standard REPLACE INTO expression.

Documentation Oracle Follows de facto standard.

Documentation Informix On my TODO.

Note 1:

In this author's opinion, it's confusing that most (if not all) string-related functions in MySQL work case sensitively, while MySQL's default behaviour is to work case insensitively in plain WHERE-clauses involving string comparisons.

Standard Core SQL feature ID E021-09: TRIM( where characters FROM string_to_be_trimmed ) where may be one of LEADING, TRAILING or BOTH—or omitted which implies BOTH. characters indicates what character(s) to remove from the head and/or tail of the string. It may be omitted which implies the value ' ' (space character). In other words, the shortest form is TRIM(string_to_be_trimmed) which in effect means TRIM(BOTH ' ' FROM string_to_be_trimmed) . Trimming NULL returns NULL. PostgreSQL Follows the standard. Documentation DB2 Follows the standard.(since version 9.1) In db2 versions lower than 9.1, you only have:

LTRIM(string_to_be_trimmed)

and

RTRIM(string_to_be_trimmed) Documentation. MSSQL Doesn't support the standard TRIM function. Provides

LTRIM(string_to_be_trimmed)

and

RTRIM(string_to_be_trimmed) Documentation: LTRIM and RTRIM MySQL Follows the standard. Documentation Oracle Follows the standard with two exceptions: Oracle doesn't allow you to trim multiple characters. I.e., TRIM('**' FROM foo ) is illegal in Oracle.

is illegal in Oracle. Due to Oracle's non-standard NULL-handling, you may get strange results of trimming NULL or the empty string. Documentation Informix On my TODO.

It's often important to get the value of current date and time. Below are the functions used to do that in the different implementations.

The current timestamp (without time zone) is retrieved with the LOCALTIMESTAMP function which may be used as: SELECT LOCALTIMESTAMP ...

or

SELECT LOCALTIMESTAMP(precision) ... Note that " SELECT LOCALTIMESTAMP() ... " is illegal: If you don't care about the precision, then you must not use any parenthesis. If the DBMS supports the non-core time zone features (feature ID F411), then it must also provide the functions CURRENT_TIMESTAMP and CURRENT_TIMESTAMP(precision) which return a value of type TIMESTAMP WITH TIME ZONE. If it doesn't support time zones, then the DBMS must not provide a CURRENT_TIMESTAMP function. Follows the standard. Documentation Doesn't have the LOCALTIMESTAMP function. Instead, it provides a special, magic value ('special register' in IBM language), CURRENT_TIMESTAMP (alias to 'CURRENT TIMESTAMP') which may be used as though it were a function without arguments. However, since DB2 doesn't provide TIMESTAMP WITH TIME ZONE support, the availability of CURRENT_TIMESTAMP could be said to be against the standard—at least confusing. Documentation Doesn't have the LOCALTIMESTAMP function. Instead, it has CURRENT_TIMESTAMP which—however—doesn't return a value of TIMESTAMP WITH TIME ZONE, but rather a value of MSSQL's DATETIME type (which doesn't contain time zone information). Documentation Follows the standard. Documentation Follows the standard. On my TODO.

Standard Core feature ID E021-07:

Concatenating two strings is done with the || operator: string1 || string2 If at least one operand is NULL, then the result is NULL. It's unclear to me if the DBMS is allowed to try to automatically cast the operands to concatenation-compatible types. PostgreSQL Follows the standard. Automatically casts the concatenated values into types compatible with concatenation. If an operand is NULL then the result is NULL. Documentation DB2 Follows the standard, partly. Does not automatically cast concatenated values into compatible types. Documentation MSSQL Breaks the standard by using the '+' operator instead of '||'. Does not automatically cast operands to compatible types. If an operand is NULL, then the result is NULL. Documentation MySQL Badly breaks the standard by redefining || to mean OR . Offers instead a function, CONCAT(string, string) , which accepts two or more arguments. Automatically casts values into types which can be concatenated. If an operand is NULL, then the result is NULL. Documentation Oracle Follows the standard, partly. Automatically casts values into types which can be concatenated. As Oracle interprets NULL as the empty string, it doesn't return NULL if an operand is NULL. Documentation Informix Follows the standard. Automatically casts numeric data into character data, if needed. If an operand is NULL then the result is NULL. Documentation

Standard As the constraint name indicates, a (set of) column(s) with a UNIQUE constraint may only contain unique (combinations of) values. A column—or a set of columns—which is subject to a UNIQUE constraint must also be subject to a not NULL constraint, unless the DBMS implements an optional "NULLs allowed" feature (Feature ID 591). The optional feature adds some additional characteristics to the UNIQUE constraint: Columns involved in a UNIQUE constraint may also have NOT NULL constraints, but they do not have to. If columns with UNIQUE constraints do not also have NOT NULL constraints, then the columns may contain any number of NULL-'values'. (Logical consequence of the fact that NULL<>NULL.)

In the standard-parlance, the constraint is satisfied, if there are no two rows in [the relation] such that the value of each column in one row is non-null and is not distinct from the value of the corresponding column in the other row PostgreSQL Follows the standard, including the optional NULLs allowed feature. Documentation DB2 Follows the non-optional parts of the UNIQUE-constraint. Doesn't implement the optional NULLs allowed feature. Documentation (see the unique-constraint section of the page). MSSQL Follows the standard—with a twist: MSSQL offers the NULLs allowed feature, but allows at most one instance of a NULL-'value', if NULLs are allowed; i.e. breaks characteristic 2 in the above description of the standard. Documentation MySQL Follows the standard, including the optional NULLs allowed feature. Oracle Follows the standard—with a twist regarding multiple-column UNIQUE-constraints: The optional NULLs allowed feature is implemented: If the UNIQUE-constraint is imposed on a single column, then the column may contain any number of NULLs (as expected from characteristic 2 in the above description of the standard). However, if the UNIQUE-constraint is specified for multiple columns, then Oracle sees the constraint as violated if any two rows contain at least one NULL in a column affected by the constraint

identical, non-NULL values in the rest of the columns affected by the constraint Documentation Informix On my TODO.

It's sometimes handy to have the DBMS handle generation of keys. The DBMSes offer various means for this. Note, however, that some database authorities warn against—at least some variants of—auto-generated keys; this is a classic database discourse.

Standard The standard specifies a column attribute of:

GENERATED ... AS IDENTITY (non-core feature ID T174+T175). When creating a table, an IDENTITY clause may be declared for certain types of columns (INTEGER being one): CREATE TABLE tablename (

tablename_id INTEGER GENERATED ALWAYS AS IDENTITY

...

) or CREATE TABLE tablename (

tablename_id INTEGER GENERATED BY DEFAULT AS IDENTITY

...

) The column with the IDENTITY attribute will be given values in increasing order, possibly with 'holes' (...,3,4,7,...). A base table may at most contain one column with the IDENTITY attribute. NOT NULL is implied for an IDENTITY column. Normally, a column declared with IDENTITY will also be declared PRIMARY KEY, but it's not implied. The examples differ in their 'ALWAYS' vs. 'BY DEFAULT' clauses: When ALWAYS is specified, the user cannot specify a value for the column which means that the DBMS can guarantee successful insertion of a unique value on each table insert.

When BY DEFAULT is specified, the user may manually specify what value to put in the identity field of a row. The flip side is that the DBMS cannot guarantee that this will work. The standard specifies several extended options which may be declared for a generated IDENTITY column. PostgreSQL PostgreSQL doesn't support the standard's IDENTITY attribute. PostgreSQL's best offering for a column with auto-generated values is to declare a column of 'type' SERIAL: CREATE TABLE tablename (

tablename_id SERIAL,

...

) 'SERIAL' is a short-hand for creating a sequence and using that sequence to create unique integers for a column. If the table is dropped, PostgreSQL will drop the sequence which was created as a side-effect of using the SERIAL type. As a user may manually insert or update a value in a column created as SERIAL, this comes closest to the standard's GENERATED BY DEFAULT AS IDENTITY variant. If you want semantics like the standard's GENERATED ALWAYS AS IDENTITY , then SERIAL will not do it; instead you need to: Create a sequence for the table (assuming that the table is called footab, having a an integer column called id):

CREATE SEQUENCE footab_id_seq Add the PL/pgSQL language to the database, in case it doesn't already exist (extra additions don't hurt):

CREATE LANGUAGE plpgsql Create a function to be called by a trigger when footab is changed:

CREATE OR REPLACE FUNCTION protect_footab_id() RETURNS TRIGGER AS $$

BEGIN

IF tg_op = 'INSERT' THEN

IF new.id IS NOT NULL THEN

RAISE EXCEPTION 'setting ID manually not allowed (%)', new.id;

END IF;

new.id = NEXTVAL('footab_id_seq');

ELSE

IF new.id IS DISTINCT FROM old.id THEN

RAISE EXCEPTION 'changing ID is not allowed (% to %)', old.id, new.id;

END IF;

END IF;

RETURN NEW;

END;

$$ LANGUAGE PLPGSQL Create the trigger, calling the above function:

CREATE TRIGGER protect_footab_id

BEFORE INSERT OR UPDATE ON footab

FOR EACH ROW EXECUTE PROCEDURE protect_footab_id() Another option is to add the WITH OIDS clause when creating a table. Object identifiers (OIDs) will then be added to a special oid column which is hidden by default, i.e. isn't included in SELECT * FROM ... result sets). The oid column can be revealed by explicitly adding it to the SELECT -list, and it can be referred to in WHERE clauses. OIDs cannot be assigned by the user, so the semantics of OIDs resemble the standard's GENERATED ALWAYS AS IDENTITY attribute. Documentation: The SERIAL and OIDs types. DB2 Follows standard, albeit with some restrictions on how identity columns may (not) be added to an existing table, etc. Documentation: CREATE TABLE syntax and description of identity columns. MSSQL MSSQL offers IDENTITY as a column property, but with a different syntax than the standard's specification. An example of creating a table with an IDENTITY column: CREATE TABLE tablename (

tablename_id INT IDENTITY PRIMARY KEY,

...

) With MSSQL's IDENTITY attribute, the user cannot manually insert the value, unless the user has first run SET IDENTITY_INSERT tablename ON

MSSQL refuses to update values in IDENTITY columns. I.e., MSSQL's IDENTITY type is closest to the standard's GENERATED ... ALWAYS AS IDENTITY variant. Documentation: The IDENTITY property and SET IDENTITY_INSERT. MySQL MySQL doesn't support the standard's IDENTITY attribute. As an alternative, an integer column may be assigned the non-standard AUTO_INCREMENT attribute: CREATE TABLE tablename (

tablename_id INTEGER AUTO_INCREMENT PRIMARY KEY,

...

) Columns with the AUTO_INCREMENT attribute will—under certain conditions—automatically be assigned a value of <largest value in column>+<at least 1>. Look in MySQL's documentation for the (rather extensive) details. A table can have at most one column with the AUTO_INCREMENT attribute; that column must be indexed (it doesn't have to be a primary key, as in the example SQL above) and cannot have a DEFAULT value attribute. It's probably not too far fetched to think of MySQL's AUTO_INCREMENT feature as this equivalence:

MySQL:

CREATE TABLE tablename (

columnname INTEGER AUTO_INCREMENT PRIMARY KEY

...

)



Standard SQL:

CREATE TABLE tablename (

columnname INTEGER DEFAULT some_func() PRIMARY KEY

...

)

where some_func() is a function which finds 1 plus the currently largest value of columnname. The nice thing about this approach is that the automatic value insertion should never fail, even though some of the column's values might have been manually set—i.e. the combined advantages of the standard's ALWAYS and BY DEFAULT variants. The drawback is that it might result in more house-keeping: The system may need extra table locks when performing row updates/insertions to protect against ghost updates in concurrent transactions—thus slowing down the system in case of many concurrent updates/insertions. Documentation Oracle Oracle doesn't support the standard's IDENTITY attribute. If you want an auto-incrementing column in Oracle, then create a sequence and use that sequence in a trigger associated to the table. Example: For the table mytable, you want the mytable_id column to be of integer type, with an auto-incrementing values: CREATE TABLE mytable (

mytable_id INTEGER PRIMARY KEY,

... -- (other columns)

);



CREATE SEQUENCE mytable_seq;



CREATE TRIGGER mytable_seq_trigger

BEFORE INSERT ON mytable FOR EACH ROW

BEGIN

IF (:new.mytable_id IS NULL) THEN

SELECT mytable_seq.nextval INTO :new.mytable_id

FROM DUAL;

END IF;

END;

/ This will create an auto-incrementing column resembling the GENERATED BY DEFAULT variant from the standard. If an column resembling the GENERATED ALWAYS variant is needed, then the trigger should be extended to raise an exception if the user tries to insert a non-NULL value, and a trigger preventing UPDATEs of the relevant column should be added. Note: If 'nice', incrementing values aren't important, you may use Oracle's SYS_GUID function as the default for a column; that way, universally unique identifiers will be assigned if you don't indicate a value for the column in new rows. Documentation: CREATE TRIGGER , CREATE SEQUENCE , and SYS_GUID . Informix On my TODO.

Note: IBM has a page comparing IDENTITY columns and sequences.

Often, it's useful to be able to remove all rows from a large table in a quick way. And often, DELETE isn't as quick as you'd like it to be. So several DBMSes implement a TRUNCATE operation. Typically, truncating means that deletion isn't associated with triggers which may exist for the table, and typically, truncating involves little (if any) transaction log activity.

Standard The SQL standard defines the TRUNCATE TABLE tablename statement (optional feature ID F200, new in SQL:2008) as:

Delete all rows of a base table without causing any triggered action. Unfortunately, the standard doesn't specify whether TRUNCATE TABLE should be allowed in a transaction involving other statements, or not whether TRUNCATE TABLE should imply an immediate COMMIT , or not PostgreSQL Follows the standard. In PostgreSQL, TRUNCATE TABLE is allowed in a transaction involving other operations, and TRUNCATE TABLE does not imply an immediate COMMIT operation. See the documentation for variations and restrictions. Most importantly, you need to have be owner of the table to be truncated (or work as a superuser); alternatively, you need to have TRUNCATE privilege on the table. Note also the nice—but potentially dangerous— CASCADE modifier which may be useful for emptying related tables. Documentation DB2 Almost follows the standard.(since version 9.7)

DB2 requires that the IMMEDIATE keyword be added the the ordinary TRUNCATE TABLE statement, e.g.:

TRUNCATE TABLE someschema.sometable IMMEDIATE

TRUNCATE TABLE must be the first statement in a transaction. A transaction starting with TRUNCATE TABLE may include other statements, but if the transaction is rolled back, the TRUNCATE TABLE operation is not undone.

DB2s TRUNCATE TABLE operation has a number of optional arguments, see the documentation for more on this; especially, the REUSE STORAGE argument may be important for ad-hoc DBA tasks. In DB2 versions < 9.7, you may abuse the IMPORT statement. Unfortunately, you need to know which operating system the command is executed from for this to work: On unix-like systems:

IMPORT FROM /dev/null OF DEL REPLACE INTO tablename

On Windows:

IMPORT FROM NUL OF DEL REPLACE INTO tablename IMPORT cannot be abused in all contexts. E.g., when working with dynamic SQL (from Java/.NET/PHP/...—not using the db2 command line processor), you need to wrap the IMPORT command in a call to ADMIN_CMD , e.g.:

CALL ADMIN_CMD('IMPORT FROM /dev/null OF DEL REPLACE INTO tablename') IMPORT seems to be allowed in a transaction involving other operations, however it implies an immediate COMMIT operation. The ALTER TABLE command may also be abused to quickly empty a table, but it requires more privileges, and may cause trouble with rollforward recovery. Documentation: TRUNCATE TABLE

IMPORT

IMPORT through ADMIN_CMD MSSQL Follows the standard. In MSSQL, TRUNCATE TABLE is allowed in a transaction involving other operations, and TRUNCATE TABLE does not imply an immediate COMMIT operation. You need to have at least ALTER-permission on the table to be truncated. Documentation MySQL MySQL has a TRUNCATE TABLE statement, but it doesn't always follow the standard. Note that in some cases, MySQL's truncate command is really the equivalent of an unrestricted DELETE command (i.e.: potentially slow and trigger-invoking). Its behaviour depends on which storage engine the table is managed by. When using InnoDB (transaction safe) tables, TRUNCATE TABLE is allowed in a transaction involving other operations, however TRUNCATE TABLE implies an immediate COMMIT operation. Documentation Oracle Follows the standard. Note, that the TRUNCATE TABLE implicitly commits the current transaction. You may find that TRUNCATE TABLE isn't nearly as quick as expected; in this case, consider using TRUNCATE TABLE tablename REUSE STORAGE instead. Needed privileges—Quoting from the documentation:

...the table or cluster must be in your schema or you must have DROP ANY TABLE system privilege. Documentation Informix On my TODO.

The following are not necessarily SQL operations, but rather a description of how different operations are performed in the command line interface provided by each product.

The shape of the command line interfaces in the commercial products is depressing. Vendors, please do something about it: Not all database developers like to use slow GUIs for technical stuff. And sometimes, DBMS work is performed over slow Internet lines which makes a decent command line interface vital.

Fortunately, a tool like HenPlus exists. It can be a pain to install, but once working, it's nice to work with.

Standard Not defined. PostgreSQL Run:

psql

which should be in the PATH in any sensible installation. PostgreSQL's command line interface is very user friendly. It has command history (press arrow-up for previous commands) and a fairly well-working command completion feature. Documentation DB2 Run:

db2 -t

(The -t argument tells the command line processor to a semicolon as statement terminator instead of the default (newline). This allows for multi-line SQL statements.) The db2 binary may not be in your PATH or may be missing vital environment variables (which is one of the stupid parts of DB2's installation procedure: It doesn't offer to set up a proper global DB2 environment for the users on the server) and you may have to include the db2profile file (situated in the sqllib directory in the home directory of the special DB2 instance user) into your shell.

E.g. on my Linux system, I've added the following line to my .bash_profile in order to get a shell with proper DB2 environment when logging in:

. /home/db2inst1/sqllib/db2profile The 'utility' doesn't seem to have anything resembling useful command history or command completion. Fortunately, queries may be sent to the db2 'utility' in a non-interactive way like this:

db2 "SELECT a_column FROM a_table"

This allows you to make use of your shell's command history handling. DB2 also has a 'utility' called db2batch which some might find at bit nicer to work with. Documentation MSSQL The command line interface is started by running

sqlcmd sqlcmd is not nice to work with. It's bad at formatting result sets. It doesn't have command line completion. You have to say go after your commands. A positive thing about sqlsmd: It has command history, so you may press arrow-up for previous commands in the current sqlsmd session. In MSSQL 2000, the command line interface was started by running osql . An alternative to osql—apart from HenPlus, mentioned above—is SQSH which should work on any modern open source operating system, except it doesn't seem to support Kerberos, so you need to log into the database using a database-account (not a Windows-account). Documentation MySQL Run:

mysql If you need help on the optional command line options, see the man page. On platforms like Linux and FreeBSD (which have decent readline-capabilities), MySQL's command line interface is simply great; not much else to say. MySQL's command line interface is said to be rather poor on Windows, though. Oracle Run:

sqlplus sqlplus lacks command completion, and has very limited built-in command history handling. The bad command history handling is especially bad on Linux/unix; fortunately, this can be fixed using rlwrap. Documentation A unique feature of Oracle is that a web-based administration interface is provided, as a supplement to the local administration software. The URL to the interface is typically in version 11: https:// hostname :1158/em/

in version 12: https:// hostname :5500/em/ Informix Informix' command line utility is called dbaccess . If run without arguments, it starts a menu system. If you simply want to shoot SQL statements off to the database, another form is more convenient, at least on unix:

echo 'SELECT foo FROM bar' | dbaccess databasename Documentation

Standard Not specified, as far as I know. (By the way: The SQL standard doesn't have the concept of a database as a container of schemas; instead, the standard specifies that schemas are contained in a catalog.) PostgreSQL Using SQL: SELECT datname FROM pg_catalog.pg_database When working in the psql command line interface: \l or \l+ Alternative (when working from the terminal, not in psql ): psql --list Documentation: The psql tool, the pg_database catalog. DB2 Offers the LIST DATABASE DIRECTORY command, but only when working in the db2 command line processor (i.e. not when working from db2batch ); this command's output is human readable, but sub-optimal as machine readable format. Documentation MSSQL EXEC SP_HELPDB Documentation MySQL SHOW DATABASES Documentation Oracle In Oracle, there is a one-to-one relationship between databases and instances (unless you work with a clustered Oracle system). You can get a list of instances; the way to do it depends on the operating system which Oracle is running on: On unix-like systems: Look in the /etc/oratab file.

file. On Windows: Start Windows' Services management console and look for services with names starting with OracleServiceXXXX . Each XXXX is the name (AKA SID) of an instance. Documentation: oratab Informix Connect to the sysmaster database (all users are allowed to do this) and run:

SELECT name FROM sysmaster:sysdatabases Documentation

Standard SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA PostgreSQL In the command line interface: \dn or \dn+ (for more details). Using SQL: Follows the standard. Documentation: The psql tool

tool The schemata INFORMATION_SCHEMA view DB2 SELECT schemaname FROM syscat.schemata Documentation MSSQL Follows the standard. Documentation MySQL MySQL doesn't support schemas. Oracle Oracle has a peculiar approach to schemas: A schema exists for each and every user. And there cannot be a schema without a corresponding user. Consequently, a way to get a list of schemas in Oracle is to query the ALL_USERS dictionary view:

SELECT username FROM all_users Documentation Informix Informix' concept of schemas is closely related to user names, so—somewhat surprisingly—the query is:

SELECT UNIQUE owner FROM systables WHERE tabid>99 Documentation

Standard Part 11 of the SQL standard specifies the INFORMATION_SCHEMA schema which must be part of all database catalogues. The schema may be used like this: SELECT * FROM INFORMATION_SCHEMA.TABLES

WHERE TABLE_TYPE='BASE TABLE' or (often more relevant): SELECT * FROM INFORMATION_SCHEMA.TABLES

WHERE TABLE_TYPE='BASE TABLE'

AND TABLE_SCHEMA='SCHEMA-NAME' See a warning about potential case sensitivity problems below. PostgreSQL Follows the standard, except for some gotchas mentioned below. In command-line context, it's easier to use the following non-SQL command instead of querying the INFORMATION_SCHEMA:

\dt Documentation: The tables INFORMATION_SCHEMA view, the psql tool. DB2 Doesn't provide the standard INFORMATION_SCHEMA . Instead, DB2 offers the SYSCAT schema (catalog) which is somewhat compatible. Offers what is probably a shorthand to some system catalog query:

LIST TABLES

or - if you want to see tables in another schema:

LIST TABLES FOR SCHEMA foo

These commands are only available in the db2 command line processor (i.e. not from—e.g.— db2batch ). Documentation MSSQL Follows that standard.

Sometimes, the SP_TABLES system stored procedure is easier to use. Documentation: The INFORMATION_SCHEMA.TABLES view

sp_tables MySQL Follows the standard, except that MySQL doesn't support schemas, so one might say that MySQL's INFORMATION_SCHEMA is really an 'INFORMATION_DATABASE' or 'INFORMATION_CATALOGUE'. In command-line context, it's easier to use the following non-standard SQL:

SHOW TABLES Documentation: The INFORMATION_SCHEMA

SHOW TABLES Oracle Doesn't provide the standard INFORMATION_SCHEMA. Provides a data dictionary system instead. The quickest way to get a usable list of 'normal' tables in the current schema:

SELECT * FROM tab

Use of the tab dictionary view is officially deprecated, though. The following query takes longer to write, but is more future proof:

SELECT owner||'.'||table_name FROM all_all_tables

(Remember that in Oracle, there is a one-to-one relationship between 'owners' and schemas.) Documentation Informix Doesn't provide the standard INFORMATION_SCHEMA out of the box. A few of the standard's INFORMATION_SCHEMA views may be added by running a special script, though. Informix offers a set of system catalogs instead. To get a list of tables:

SELECT tabname FROM systables WHERE tabid > 99 The above query will include views and other objects; if you want base tables only:

SELECT tabname FROM systables WHERE tabid > 99 AND tabtype='T' Documentation

Warning about a general case sensitivity gotcha Note that there may be case sensitivity issues involved when using meta-data views like those in the INFORMATION_SCHEMA. Generally, the standard states that the name of an identifier (such as table names) are implicitly converted to uppercase, unless double-quotes are used when referring to the identifier. The same goes for identifiers used in queries: A query like SELECT foo FROM tablename is implicitly converted to SELECT FOO FROM TABLENAME . If you create your table as

CREATE TABLE testtab (id INTEGER PRIMARY KEY)

then a query like

SELECT * FROM testtab

should work fine, and

SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='TESTTAB'

should work, while the following query will probably fail:

SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='testtab'

Warning about INFORMATION_SCHEMA gotchas in PostgreSQL Warning: PostgreSQL's case-conversion rules for unquoted identifiers (such as table names) are non-standard: PostgreSQL converts the identifiers to lower case, instead of converting to upper case. This means that you may try altering the case of identifier names used for queries in the INFORMATION_SCHEMA if you experience unexpected, empty metadata queries. Note also that due to PostgreSQL's handling of constraint names, the INFORMATION_SCHEMA cannot safely be used to deduce referential constraints; for this, you have to use PostgreSQL's pg_catalog system-schema.

Standard Part 11 of the SQL standard specifies the INFORMATION_SCHEMA schema which must be part of all database catalogues. The schema may be used like this: SELECT column_name,data_type,column_default,is_nullable

FROM

information_schema.tables AS t

JOIN

information_schema.columns AS c ON

t.table_catalog=c.table_catalog AND

t.table_schema=c.table_schema AND

t.table_name=c.table_name

WHERE

t.table_name='TABLE-NAME' —or like this (more verbose): SELECT

column_name,

data_type,

character_maximum_length,

numeric_precision,

column_default,

is_nullable

FROM

information_schema.tables as t

JOIN

information_schema.columns AS c ON

t.table_catalog=c.table_catalog AND

t.table_schema=c.table_schema AND

t.table_name=c.table_name

WHERE

c.table_schema='TABLE-SCHEMA'

AND

c.table_name='TABLE-NAME' To get information about constraints, involved columns and (possibly) referenced columns, a query like this may be used:

SELECT

tc.CONSTRAINT_NAME,

CONSTRAINT_TYPE,

ccu.COLUMN_NAME,

rccu.COLUMN_NAME,

rccu.TABLE_CATALOG,

rccu.TABLE_SCHEMA,

rccu.TABLE_NAME,

CHECK_CLAUSE

FROM

INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc

LEFT JOIN

INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE ccu ON

tc.CONSTRAINT_CATALOG=ccu.CONSTRAINT_CATALOG AND

tc.CONSTRAINT_SCHEMA=ccu.CONSTRAINT_SCHEMA AND

tc.CONSTRAINT_NAME=ccu.CONSTRAINT_NAME AND

tc.TABLE_CATALOG=ccu.TABLE_CATALOG AND

tc.TABLE_SCHEMA=ccu.TABLE_SCHEMA AND

tc.TABLE_NAME=ccu.TABLE_NAME

LEFT JOIN

INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS rc ON

rc.CONSTRAINT_CATALOG=ccu.CONSTRAINT_CATALOG AND

rc.CONSTRAINT_SCHEMA=ccu.CONSTRAINT_SCHEMA AND

rc.CONSTRAINT_NAME=ccu.CONSTRAINT_NAME

LEFT JOIN

INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE rccu ON

rc.UNIQUE_CONSTRAINT_CATALOG=rccu.CONSTRAINT_CATALOG AND

rc.UNIQUE_CONSTRAINT_SCHEMA=rccu.CONSTRAINT_SCHEMA AND

rc.UNIQUE_CONSTRAINT_NAME=rccu.CONSTRAINT_NAME

LEFT JOIN

INFORMATION_SCHEMA.CHECK_CONSTRAINTS cc ON

tc.CONSTRAINT_CATALOG=cc.CONSTRAINT_CATALOG AND

tc.CONSTRAINT_SCHEMA=cc.CONSTRAINT_SCHEMA AND

tc.CONSTRAINT_NAME=cc.CONSTRAINT_NAME

WHERE

tc.TABLE_CATALOG='CATALOG-NAME' AND -- see remark

tc.TABLE_SCHEMA='SCHEMA-NAME' AND -- see remark

tc.TABLE_NAME='TABLE-NAME'

ORDER BY tc.CONSTRAINT_NAME If you don't care about potential namespace conflicts, you may leave out the lines commented with " -- see remark ". See also: Warning about potential case sensitivity problems above. PostgreSQL Follows the standard, except for some gotchas mentioned above. In command-line context it's easier to use this non-SQL command:

\d tablename DB2 Doesn't provide the standard INFORMATION_SCHEMA. To obtain (very) basic information about a table:

DESCRIBE TABLE tablename

DESCRIBE INDEXES FOR TABLE tablename SHOW DETAIL To get information about constraints, including involved/referred columns, a query like the following may be used, although the db2 'utility' isn't good at adjusting column widths in output (i.e. the output is not easy to read):

SELECT

tc.constname as const_name,

type as const_type,

kcu.colname as col_name,

r.reftabschema as ref_tabschema,

r.reftabname as ref_tabname,

kcu_r.colname as ref_colname

FROM

syscat.tabconst tc

JOIN

syscat.keycoluse kcu ON

tc.constname=kcu.constname

LEFT JOIN

syscat.references r ON

type='F' AND

tc.constname=r.constname

LEFT JOIN

syscat.keycoluse kcu_r ON

r.constname=kcu_r.constname

WHERE

tc.tabschema=UCASE('schemaname') AND

tc.tabname=UCASE('tablename')

ORDER BY const_name,col_name Documentation: The DESCRIBE command in the "db2" command line processor

SYSCAT views MSSQL Follows the standard, except that MSSQL uses non-standard names for some standard datatypes, i.e. varchar instead of the standard's CHARACTER_VARYING

MSSQL's INFORMATION_SCHEMA doesn't have all SQL:2008's columns (an example: MSSQL's INFORMATION_SCHEMA.COLUMNS view does not contain the IS_IDENTITY column) Often, the SP_HELP 'tablename' system stored procedure is easier to use. Documentation: Information Schema Views

sp_help MySQL Follows the standard, except that MySQL doesn't support schemas, so one might say that MySQL's INFORMATION_SCHEMA is really an 'INFORMATION_DATABASE' or 'INFORMATION_CATALOGUE'.

is really an 'INFORMATION_DATABASE' or 'INFORMATION_CATALOGUE'. MySQL's INFORMATION_SCHEMA doesn't have all SQL:2008's columns (an example: MySQL's INFORMATION_SCHEMA.COLUMNS view does not contain the IS_IDENTITY column).

As MySQL's namespaces don't match the SQL standard fully, the standard queries mentioned above will not work. The reason is that in MySQL, the value of TABLE_CATALOG is NULL for all tables and columns. To obtain the wanted information, you need to remove the table_catalog join-conditions. I.e., the first (and simplest) of the above queries must be re-written to: SELECT column_name,data_type,column_default,is_nullable

FROM

information_schema.tables AS t

JOIN

information_schema.columns AS c ON

t.table_schema=c.table_schema AND

t.table_name=c.table_name

WHERE

t.table_name='TABLE-NAME' In command-line context it's easier to use this non-SQL command:

DESCRIBE tablename Documentation: The INFORMATION_SCHEMA

DESCRIBE Oracle Doesn't provide the standard INFORMATION_SCHEMA. Offers data dictionary views instead. To get (very) basic information:

DESCRIBE tablename To get information on constraints, including foreign (referred) table/column information, a query like this may be used (adjust tablename in one of the last lines):

COLUMN consname FORMAT a11;

COLUMN colname FORMAT a10;

COLUMN type FORMAT a11;

COLUMN cond FORMAT a20;

COLUMN ref_tabname FORMAT a11;

COLUMN ref_colname FORMAT a11;

SELECT

uc.constraint_name consname,

ucc.column_name colname,

CASE

WHEN uc.constraint_type='C' THEN 'CHECK'

WHEN uc.constraint_type='P' THEN 'PRIMARY KEY'

WHEN uc.constraint_type='R' THEN 'REFERENTIAL'

WHEN uc.constraint_type='U' THEN 'UNIQUE'

ELSE uc.constraint_type

END as type,

uc.search_condition cond,

ucc_r.table_name ref_tabname,

ucc_r.column_name ref_colname

FROM

user_constraints uc

JOIN

user_cons_columns ucc ON

uc.constraint_name=ucc.constraint_name AND

uc.owner=ucc.owner

LEFT JOIN

user_constraints uc_r ON

uc.r_constraint_name=uc_r.constraint_name AND

uc.owner=uc_r.owner

LEFT JOIN

user_cons_columns ucc_r ON

uc_r.constraint_name=ucc_r.constraint_name AND

uc_r.owner=ucc_r.owner

WHERE

uc.TABLE_NAME = UPPER('tablename')

ORDER BY consname,colname ; To get information on indexes on a table, a query like this may be used (adjust tablename in one of the last lines):

COLUMN index_name FORMAT a11;

COLUMN type FORMAT a8;

COLUMN uniness FORMAT a9;

COLUMN column_name FORMAT a20;

SELECT index_name,

index_type type,

uniqueness uniness,

column_name

FROM user_indexes ui

NATURAL JOIN user_ind_columns uic

WHERE dropped='NO'

AND table_name=upper('tablename')

ORDER BY index_name,column_name Documentation: DESCRIBE sqlplus command

COLUMN sqlplus command

Static Data Dictionary Views

USER_CONSTRAINTS data dictionary view

USER_CONS_COLUMNS data dictionary view

USER_INDEXES data dictionary view

USER_IND_COLUMNS data dictionary view Informix Doesn't provide the standard INFORMATION_SCHEMA out of the box. If a special script is run, an INFORMATION_SCHEMA may be added which allows for using the most basic standards-based table description query. In practice, an Informix-only query is used. The following query provides very basic table information, excluding constraints:

SELECT

colname,

coltype,

CASE

WHEN (coltype-256)<0 THEN 'YES'

ELSE 'NO'

END AS nullable

FROM systables AS a

JOIN syscolumns AS b ON a.tabid = b.tabid

WHERE tabname='tablename' Notice that the table name is in lower case. The colname values are numeric codes which need to be looked up in order to provide meaning. Documentation

In most DBMSes, it's possible to enable automatic statistics gathering, but sometimes, it's nice to be able to manually tell the DBMS to gather statistics for a table (or a number of tables).

Standard Not standardized. PostgreSQL ANALYZE tablename If the tablename parameter is left out, then statistics are gathered for all tables in the current database. Documentation DB2 RUNSTATS ON TABLE schema-name . table-name AND INDEXES ALL

(many variations/options available) The RUNSTATS command needs to be invoked in a special way if you aren't using the db2 command line processor, namely through the ADMIN_CMD procedure. Documentation: RUNSTATS and RUNSTATS wrapped in ADMIN_CMD . MSSQL First, you have to add statistics to the table:

CREATE STATISTICS stats_name

ON table_name

( column_name_1 , column_name_2 , column_name_3 , ...)

(The CREATE STATISTICS step is not needed for indexed columns. Thus, this step may be skipped if you are satisfied with keeping statistics on indexed columns only.) The statistics may then be updated when needed:

UPDATE STATISTICS table_name Having to explicitly mention tables and columns can be tedious, and in many cases, the sp_createstats and sp_updatestats stored procedures are easier to use. Documentation: CREATE STATISTICS, UPDATE STATISTICS, sp_createstats, sp_updatestats MySQL ANALYZE TABLE tablename Documentation Oracle Oracle offers to estimate (quick) or compute (thorough) statistics for a database object. The quick way to do this is to use the deprecated ANALYZE command which can be used in various ways, e.g. ANALYZE TABLE tablename ESTIMATE STATISTICS;

ANALYZE TABLE tablename ESTIMATE STATISTICS FOR ALL INDEXES;

(It's unclear to me if both are needed to gain the relevant statistics.) —Or:

ANALYZE TABLE tablename COMPUTE STATISTICS;

ANALYZE TABLE tablename COMPUTE STATISTICS FOR ALL INDEXES; If you want to stay away from deprecated features (although I doubt that Oracle will remove ANALYZE...STATISTICS... any time soon), you need to use the DBMS_STATS package. Documentation Informix On my TODO.

Standard Not standardized. PostgreSQL EXPLAIN <query> Documentation DB2 The easiest way to get a query explanation is to save the query in a file (without a terminating semicolon), and then run a special command-line utility:

db2expln -database databasename -stmtfile query.sql -terminator ';' -terminal

In the above example, the query has been saved to a file called "query.sql". In some situations, you may want to use the dynexpln utility instead of db2expln . And in yet other situations, the db2exfmt tool is a better choice. A visual explanation tool also exists. If you prefer to get the explanation through SQL: Set up needed explain tables using EXPLAIN.DDL which should exist in sqllib/misc of your DB2 instance user's home directory. Optionally: Clean up old plan explanations: DELETE FROM EXPLAIN_INSTANCE Generate the explanation: EXPLAIN PLAN FOR <SQL-statement> Display plan:

SELECT O.Operator_ID, S2.Target_ID, O.Operator_Type,

S.Object_Name, CAST(O.Total_Cost AS INTEGER) Cost

FROM EXPLAIN_OPERATOR O

LEFT OUTER JOIN EXPLAIN_STREAM S2

ON O.Operator_ID=S2.Source_ID

LEFT OUTER JOIN EXPLAIN_STREAM S

ON O.Operator_ID = S.Target_ID

AND O.Explain_Time = S.Explain_Time

AND S.Object_Name IS NOT NULL

ORDER BY O.Explain_Time ASC, Operator_ID ASC

(Adapted from recipe in SQL Tuning.) Documentation MSSQL MSSQL can be put in a query explanation mode where queries are not actually executed, but a query explanation is returned instead:

SET SHOWPLAN_TEXT ON The query explanation mode is turned off by running

SET SHOWPLAN_TEXT OFF Documentation MySQL EXPLAIN <query> Documentation Oracle EXPLAIN PLAN FOR <query>

After the query has run, do the following to get the plan explanation:

SELECT plan_table_output FROM table(dbms_xplan.display()) Documentation Informix On my TODO.

Standard Not standardized. PostgreSQL \timing Documentation DB2 Run the query in the " db2batch " command line processor; db2batch prints the elapsed time of each query. Documentation MSSQL SET STATISTICS TIME ON Documentation MySQL MySQL's command line interface prints query times by default. Oracle SET TIMING ON Documentation Informix On my TODO.

PostgreSQL jdbc:postgresql:// hostname / DBname Documentation DB2 jdbc:db2:// hostname : 50000 / DBname

or (if the database is on the local host):

jdbc:db2: DBname Documentation MSSQL jTDS' driver: jdbc:jtds:sqlserver://<server>[:<port>][/<database>][;<property>=<value>[;...]] If you need to connect to a named instance, see add the instance name like this:

jdbc:jtds:sqlserver://<server>[:<port>][/<database>];instance=INSTANCE_NAME[;<property>=<value>[;...]] MySQL jdbc:mysql://[ host ][, failoverhost ][: port ]/[ database ]?user= username &password= password Documentation Oracle jdbc:oracle:thin:@hostname:1521:instancename Informix jdbc:informix-sqli://hostname:9088/DBname:INFORMIXSERVER=instancename

Use port 1526 instead of 9088 if the Informix version is <11.

Some DBMSes let you perform a query like this:

SELECT 1+1

answering

2

With other DBMSes, you need to insert a dummy-table expression to obtain the same result:

SELECT 1+1 FROM dummy-table

Standard On my TODO. PostgreSQL No need for dummy-table. In addition, the VALUES keyword may be used to produce a simple result set, without introducing a FROM clause, e.g.

VALUES(1+1)

(Note the missing SELECT and FROM keywords). Documentation DB2 Dummy-table: SYSIBM.SYSDUMMY1 . In addition, the VALUES keyword may be used to produce a simple result set, without introducing a FROM clause, e.g.

VALUES(1+1)

(Note the missing SELECT and FROM keywords). Documentation MSSQL No need for dummy-table. MySQL No need for dummy-table, although MySQL allows you to refer to a DUAL dummy-table (for Oracle compatibility). Oracle Dummy-table: DUAL . Informix Informix requires that you include a FROM specification. In recent versions of Informix(since version 11.10), a dummy table has been included: sysmaster:sysdual . For older Informix versions, the tradition is to use code like:

SELECT ... FROM systables WHERE tabid=1

This code makes use of the fact that the systables table is guaranteed to contain a row where tabid equals 1. Documentation: The sysdual table

The systables table

Standard SELECT CHARACTER_VALUE

FROM INFORMATION_SCHEMA.SQL_IMPLEMENTATION_INFO

WHERE IMPLEMENTATION_INFO_NAME='DBMS VERSION' PostgreSQL Follows the standard. An alternative, non-standard function may be used:

SELECT VERSION() Documentation DB2 SELECT service_level FROM SYSIBMADM.ENV_INST_INFO —or run the special db2level program. Documentation: SYSIBMADM.ENV_INST_INFO and db2level MSSQL MSSQL's implementation of the IMPLEMENTATION_SCHEMA doesn't seem to include the SQL_IMPLEMENTATION_INFO view. In stead, you may use

SELECT SERVERPROPERTY('ProductVersion')

(just the version), or

SELECT @@VERSION

(verbose, harder to parse). Documentation: SERVERPROPERTY, @@VERSION MySQL MySQL's INFORMATION_SCHEMA doesn't include the SQL_IMPLEMENTATION_INFO view. Work-around:

SELECT VERSION() Documentation Oracle SELECT banner FROM v$version v$version will not reveal patch set updates. For details which include exact patch level, use the opatch command-line tool (which might exist as /u01/app/oracle/product/12.1.0/dbhome_1/OPatch/opatch, or similar):

opatch lsinventory Documentation: v$version Informix Using SQL: SELECT dbinfo('version','full') FROM systables WHERE tabid=1 From the command line:

onstat - Documentation: The dbinfo function

The onstat utility

Product Port# Notes Documentation Standard Not specified PostgreSQL 5432 For security reasons, PostgreSQL doesn't listen to non-local TCP interfaces by default. Documentation DB2 50000 MSSQL 1433 By default, MSSQL Express Edition doesn't listen for TCP connections. MySQL 3306 Oracle 1521 Documentation Informix 9088 (unencrypted) Informix versions prior to version 11, the default port was 1526.

Each DBMS has different ways to record diagnostic information (event logs).

Standard Not covered by the standard. PostgreSQL By default, PostgreSQL logs to stderr, meaning that it's highly installation specific where the dianostic information is put; on this author's system, the default ends up in /var/lib/pgsql/pgstartup.log . The default can be set to something more reasonable (such as syslog on unix, eventlog on Windows) by adjusting the log_destination configuration parameter. Documentation DB2 On unix systems, DB2s diagnostic log file is called db2diag.log and lives in the sqllib/db2dump sub-directory of the instance user's home directory. I.e., a typical full path is:

/home/db2inst1/sqllib/db2dump/db2diag.log

If the file is renamed or deleted, DB2 will create a new db2diag.log without having to be restarted. Documentation MSSQL On my TODO. MySQL On my TODO. Oracle A diagnostic directory contains a file called alert_ INSTANCE .log . The diagnostic directory is determined by the following query:

SELECT value FROM v$parameter WHERE name='background_dump_dest'

Examples of the diagnostic directory: On an Oracle 11gR2 running on Linux: /usr/local/oracle/diag/rdbms/ INSTANCE / INSTANCE /trace

On an Oracle 9.2 running on Windows: D:\oracle\admin\ INSTANCE \bdump Documentation Informix The path of the diagnostic log is defined by the MSGPATH configuration parameter. On a Linux installation, using default options:

/opt/IBM/informix/tmp/online.log Documentation

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