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String Aggregation Techniques

On occasion it is necessary to aggregate data from a number of rows into a single row, giving a list of data associated with a specific value. Using the SCOTT.EMP table as an example, we might want to retrieve a list of employees for each department. Below is a list of the base data and the type of output we would like to return from an aggregate query.

Base Data: DEPTNO ENAME ---------- ---------- 20 SMITH 30 ALLEN 30 WARD 20 JONES 30 MARTIN 30 BLAKE 10 CLARK 20 SCOTT 10 KING 30 TURNER 20 ADAMS 30 JAMES 20 FORD 10 MILLER Desired Output: DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,FORD,ADAMS,SCOTT,JONES 30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD

This article is based on a thread from asktom.oracle.com and contains several methods to achieve the desired results.

Setup

The examples in this article require the following table.

--DROP TABLE emp PURGE; CREATE TABLE emp ( empno NUMBER(4) CONSTRAINT pk_emp PRIMARY KEY, ename VARCHAR2(10), job VARCHAR2(9), mgr NUMBER(4), hiredate DATE, sal NUMBER(7,2), comm NUMBER(7,2), deptno NUMBER(2) ); INSERT INTO emp VALUES (7369,'SMITH','CLERK',7902,to_date('17-12-1980','dd-mm-yyyy'),800,NULL,20); INSERT INTO emp VALUES (7499,'ALLEN','SALESMAN',7698,to_date('20-2-1981','dd-mm-yyyy'),1600,300,30); INSERT INTO emp VALUES (7521,'WARD','SALESMAN',7698,to_date('22-2-1981','dd-mm-yyyy'),1250,500,30); INSERT INTO emp VALUES (7566,'JONES','MANAGER',7839,to_date('2-4-1981','dd-mm-yyyy'),2975,NULL,20); INSERT INTO emp VALUES (7654,'MARTIN','SALESMAN',7698,to_date('28-9-1981','dd-mm-yyyy'),1250,1400,30); INSERT INTO emp VALUES (7698,'BLAKE','MANAGER',7839,to_date('1-5-1981','dd-mm-yyyy'),2850,NULL,30); INSERT INTO emp VALUES (7782,'CLARK','MANAGER',7839,to_date('9-6-1981','dd-mm-yyyy'),2450,NULL,10); INSERT INTO emp VALUES (7788,'SCOTT','ANALYST',7566,to_date('13-JUL-87','dd-mm-rr')-85,3000,NULL,20); INSERT INTO emp VALUES (7839,'KING','PRESIDENT',NULL,to_date('17-11-1981','dd-mm-yyyy'),5000,NULL,10); INSERT INTO emp VALUES (7844,'TURNER','SALESMAN',7698,to_date('8-9-1981','dd-mm-yyyy'),1500,0,30); INSERT INTO emp VALUES (7876,'ADAMS','CLERK',7788,to_date('13-JUL-87', 'dd-mm-rr')-51,1100,NULL,20); INSERT INTO emp VALUES (7900,'JAMES','CLERK',7698,to_date('3-12-1981','dd-mm-yyyy'),950,NULL,30); INSERT INTO emp VALUES (7902,'FORD','ANALYST',7566,to_date('3-12-1981','dd-mm-yyyy'),3000,NULL,20); INSERT INTO emp VALUES (7934,'MILLER','CLERK',7782,to_date('23-1-1982','dd-mm-yyyy'),1300,NULL,10); COMMIT;

LISTAGG Analytic Function in 11g Release 2

The LISTAGG analytic function was introduced in Oracle 11g Release 2, making it very easy to aggregate strings. The nice thing about this function is it also allows us to order the elements in the concatenated list. If you are using 11g Release 2 you should use this function for string aggregation.

COLUMN employees FORMAT A50 SELECT deptno, LISTAGG(ename, ',') WITHIN GROUP (ORDER BY ename) AS employees FROM emp GROUP BY deptno; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 ADAMS,FORD,JONES,SCOTT,SMITH 30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD 3 rows selected.

For more information read the following article.

WM_CONCAT Built-in Function (Not Supported)

If you are not running 11g Release 2 or above, but are running a version of the database where the WM_CONCAT function is present, then it is a zero effort solution as it performs the aggregation for you. It is actually an example of a user defined aggregate function described below, but Oracle have done all the work for you.

COLUMN employees FORMAT A50 SELECT deptno, wm_concat(ename) AS employees FROM emp GROUP BY deptno; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,FORD,ADAMS,SCOTT,JONES 30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD 3 rows selected.

WM_CONCAT is an undocumented function and as such is not supported by Oracle for user applications (MOS Note ID 1336219.1). If this concerns you, use a User-Defined Aggregate Function described below. Also, WM_CONCAT has been removed from 12c onward, so you can't pick this option.

User-Defined Aggregate Function

The WM_CONCAT function described above is an example of a user-defined aggregate function that Oracle have already created for you. If you don't want to use WM_CONCAT , you can create your own user-defined aggregate function as described at asktom.oracle.com. Thanks to Kim Berg Hansen for some corrections in comments.

CREATE OR REPLACE TYPE t_string_agg AS OBJECT ( g_string VARCHAR2(32767), STATIC FUNCTION ODCIAggregateInitialize(sctx IN OUT t_string_agg) RETURN NUMBER, MEMBER FUNCTION ODCIAggregateIterate(self IN OUT t_string_agg, value IN VARCHAR2 ) RETURN NUMBER, MEMBER FUNCTION ODCIAggregateTerminate(self IN t_string_agg, returnValue OUT VARCHAR2, flags IN NUMBER) RETURN NUMBER, MEMBER FUNCTION ODCIAggregateMerge(self IN OUT t_string_agg, ctx2 IN t_string_agg) RETURN NUMBER ); / SHOW ERRORS CREATE OR REPLACE TYPE BODY t_string_agg IS STATIC FUNCTION ODCIAggregateInitialize(sctx IN OUT t_string_agg) RETURN NUMBER IS BEGIN sctx := t_string_agg(NULL); RETURN ODCIConst.Success; END; MEMBER FUNCTION ODCIAggregateIterate(self IN OUT t_string_agg, value IN VARCHAR2 ) RETURN NUMBER IS BEGIN SELF.g_string := self.g_string || ',' || value; RETURN ODCIConst.Success; END; MEMBER FUNCTION ODCIAggregateTerminate(self IN t_string_agg, returnValue OUT VARCHAR2, flags IN NUMBER) RETURN NUMBER IS BEGIN returnValue := SUBSTR(SELF.g_string, 2); RETURN ODCIConst.Success; END; MEMBER FUNCTION ODCIAggregateMerge(self IN OUT t_string_agg, ctx2 IN t_string_agg) RETURN NUMBER IS BEGIN SELF.g_string := SELF.g_string || ctx2.g_string; RETURN ODCIConst.Success; END; END; / SHOW ERRORS CREATE OR REPLACE FUNCTION string_agg (p_input VARCHAR2) RETURN VARCHAR2 PARALLEL_ENABLE AGGREGATE USING t_string_agg; / SHOW ERRORS

The aggregate function is implemented using a type and type body, and is used within a query.

COLUMN employees FORMAT A50 SELECT /*+ PARALLEL(2) */ deptno, string_agg(ename) AS employees FROM emp GROUP BY deptno; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,FORD,ADAMS,SCOTT,JONES 30 ALLEN,BLAKE,MARTIN,TURNER,JAMES,WARD 3 rows selected.

Specific Function

One approach is to write a specific function to solve the problems. The get_employees function listed below returns a list of employees for the specified department.

CREATE OR REPLACE FUNCTION get_employees (p_deptno in emp.deptno%TYPE) RETURN VARCHAR2 IS l_text VARCHAR2(32767) := NULL; BEGIN FOR cur_rec IN (SELECT ename FROM emp WHERE deptno = p_deptno) LOOP l_text := l_text || ',' || cur_rec.ename; END LOOP; RETURN LTRIM(l_text, ','); END; / SHOW ERRORS

The function can then be incorporated into a query as follows.

COLUMN employees FORMAT A50 SELECT deptno, get_employees(deptno) AS employees FROM emp GROUP by deptno; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,JONES,SCOTT,ADAMS,FORD 30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES 3 rows selected.

To reduce the number of calls to the function, and thereby improve performance, we might want to filter the rows in advance.

COLUMN employees FORMAT A50 SELECT e.deptno, get_employees(e.deptno) AS employees FROM (SELECT DISTINCT deptno FROM emp) e; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,JONES,SCOTT,ADAMS,FORD 30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES 3 rows selected.

Generic Function using Ref Cursor

An alternative approach is to write a function to concatenate values passed using a ref cursor. This is essentially the same as the previous example, except that the cursor is passed in making it generic, as shown below.

CREATE OR REPLACE FUNCTION concatenate_list (p_cursor IN SYS_REFCURSOR) RETURN VARCHAR2 IS l_return VARCHAR2(32767); l_temp VARCHAR2(32767); BEGIN LOOP FETCH p_cursor INTO l_temp; EXIT WHEN p_cursor%NOTFOUND; l_return := l_return || ',' || l_temp; END LOOP; RETURN LTRIM(l_return, ','); END; / SHOW ERRORS

The CURSOR expression is used to allow a query to be passed to the function as a ref cursor, as shown below.

COLUMN employees FORMAT A50 SELECT e1.deptno, concatenate_list(CURSOR(SELECT e2.ename FROM emp e2 WHERE e2.deptno = e1.deptno)) employees FROM emp e1 GROUP BY e1.deptno; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,JONES,SCOTT,ADAMS,FORD 30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES 3 rows selected.

Once again, the total number of function calls can be reduced by filtering the distinct values, rather than calling the function for each row.

COLUMN employees FORMAT A50 SELECT deptno, concatenate_list(CURSOR(SELECT e2.ename FROM emp e2 WHERE e2.deptno = e1.deptno)) employees FROM (SELECT DISTINCT deptno FROM emp) e1; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,JONES,SCOTT,ADAMS,FORD 30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES 3 rows selected.

ROW_NUMBER() and SYS_CONNECT_BY_PATH functions in Oracle 9i

An example on williamrobertson.net uses the ROW_NUMBER() and SYS_CONNECT_BY_PATH functions to achieve the same result without the use of PL/SQL or additional type definitions.

SELECT deptno, LTRIM(MAX(SYS_CONNECT_BY_PATH(ename,',')) KEEP (DENSE_RANK LAST ORDER BY curr),',') AS employees FROM (SELECT deptno, ename, ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY ename) AS curr, ROW_NUMBER() OVER (PARTITION BY deptno ORDER BY ename) -1 AS prev FROM emp) GROUP BY deptno CONNECT BY prev = PRIOR curr AND deptno = PRIOR deptno START WITH curr = 1; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 ADAMS,FORD,JONES,SCOTT,SMITH 30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD 3 rows selected.

COLLECT function in Oracle 10g

An example on oracle-developer.net uses the COLLECT function in Oracle 10g to get the same result. This method requires a table type and a function to convert the contents of the table type to a string. I've altered his method slightly to bring it in line with this article.

CREATE OR REPLACE TYPE t_varchar2_tab AS TABLE OF VARCHAR2(4000); / CREATE OR REPLACE FUNCTION tab_to_string (p_varchar2_tab IN t_varchar2_tab, p_delimiter IN VARCHAR2 DEFAULT ',') RETURN VARCHAR2 IS l_string VARCHAR2(32767); BEGIN FOR i IN p_varchar2_tab.FIRST .. p_varchar2_tab.LAST LOOP IF i != p_varchar2_tab.FIRST THEN l_string := l_string || p_delimiter; END IF; l_string := l_string || p_varchar2_tab(i); END LOOP; RETURN l_string; END tab_to_string; /

The query below shows the COLLECT function in action.

COLUMN employees FORMAT A50 SELECT deptno, tab_to_string(CAST(COLLECT(ename) AS t_varchar2_tab)) AS employees FROM emp GROUP BY deptno; DEPTNO EMPLOYEES ---------- -------------------------------------------------- 10 CLARK,KING,MILLER 20 SMITH,JONES,SCOTT,ADAMS,FORD 30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES 3 rows selected.

For more information see:

Hope this helps. Regards Tim...

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