JOINs are the first big learning curve after getting your head around SQL basics. More often than not you won’t find everything you need in one table so will need to learn which join to use when.

While there are plenty of guides to JOINs that use Venn diagrams to get the point across, I prefer something more visual.

INNER JOIN

LEFT JOIN

CROSS JOIN

UNION

UNION ALL

INNER JOIN

Use an INNER join, or just a JOIN, when you want to find the match between two tables.

You need to have a column on both tables that you join ON, and that’s where the match happens.

Any results where there is not a match are discarded.

Inner Join

select o.order_item, i.inventory_item from orders o inner join inventory i on o.order_item = i.inventory_item 1 2 3 4 5 6 7 select o . order_item , i . inventory_item from orders o inner join inventory i on o . order_item = i . inventory_item

LEFT JOIN

Use a LEFT JOIN when you want to find a match between two tables, but also show a NULL where there is no match from the right table. A RIGHT JOIN does the same but in reverse.

Like the INNER JOIN, you need a column to join ON. Unlike the INNER JOIN, a NULL is used to show there is no match between the two tables.

Left Join

select o.order_item, -- this is the left table i.inventory_item -- this is the right table from orders o left join inventory i on o.order_item = i.inventory_item 1 2 3 4 5 6 7 select o . order_item , -- this is the left table i . inventory_item -- this is the right table from orders o left join inventory i on o . order_item = i . inventory_item

CROSS JOIN

A CROSS JOIN joins everything with everything. There is no need to provide a column to join on, and it can result in a very big data set, (and a really big image so you’ll have to use your imagination when reviewing the image below).

Proceed with caution.

Cross Join

select o.order_item, i.inventory_item from orders o cross join inventory i 1 2 3 4 5 6 select o . order_item , i . inventory_item from orders o cross join inventory i

UNION

While a JOIN combines columns horizontally a UNION combines rows vertically. This is technically a set operator rather than a JOIN but as we are talking about combining datasets this is a good opportunity to introduce it.

Using a UNION combines the result of two queries into one column and removes duplicates.

If your query has multiple columns, they need to be in the same order to complete the UNION.

Union

select * from orders union select * from inventory 1 2 3 4 5 6 7 select * from orders union select * from inventory

UNION ALL

The UNION ALL combines the results of two queries the same as a UNION but keeps the duplicates in the result.

Union All

select * from orders union all select * from inventory 1 2 3 4 5 6 7 select * from orders union all select * from inventory

The best way to get your head around JOINs is to start using them. If you aren’t working with a SQL database already, check out SQLZoo or Hacker Rank to experiment with JOINs.

Photo by Ann H from Pexels

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