Question

Simplified for example, I have two tables, groups and items.

items (     id,     groupId,     title )  groups (     id,     groupTitle,     externalURL ) 

The regular query I’m goes something like this:

SELECT     i.`id`,     i.`title`,     g.`id` as 'groupId',     g.`groupTitle`,     g.`externalURL` FROM     items i INNER JOIN groups g ON (i.`groupId` = g.`id`) 

However I need to modify this now, because all the groups which specify an externalURL will not have any corresponding records in the items table (since they’re stored externally). Is it possible to do some sort of join so that the output looks kinda like this:

items: id    title    groupId ---------------------- 1     Item 1   1 2     Item 2   1  groups id    groupTitle    externalURL ------------------------------- 1     Group 1       NULL 2     Group 2       something 3     Group 3       NULL  Query output: id    title    groupId    groupTitle    externalURL --------------------------------------------------- 1     Item 1   1          Group 1       NULL 2     Item 2   1          Group 1       NULL NULL  NULL     2          Group 2       something  -- note that group 3 didn't show up because it had no items OR externalURL 

Is that possible in one SQL query?

Answer 1

This is exactly what an outer join is for: return all the rows from one table, whether or not there is a matching row in the other table. In those cases, return NULL for all the columns of the other table.

The other condition you can take care of in the WHERE clause.

SELECT     i.`id`,     i.`title`,     g.`id` as 'groupId',     g.`groupTitle`,     g.`externalURL` FROM     items i RIGHT OUTER JOIN groups g ON (i.`groupId` = g.`id`) WHERE i.`id` IS NOT NULL OR g.`externalURL` IS NOT NULL; 

Only if both i.id and g.externalURL are NULL, then the whole row of the joined result set should be excluded.