Question

I have two tables that are something like this:

Main table: id (int), title (varchar), etc. Sub-table: main_table_id (foreign key into main table), tag (varchar), etc.

There can be zero or more subtable rows for a given row in the main table.

I want to do a query that will return every row of the main table, with the columns of the main table, and a the columns from only a single row (doesn’t matter which) of the sub-table, if there are any, otherwise NULL in those columns.

Of course if I just do a basic LEFT OUTER JOIN then of course I get the main table repeated multiple times, one for each match in the sub-table.

I’m sure I have seen this done before using a LEFT OUTER JOIN and some sort of trickery that forces only one row to be selected from the sub-table, not all of them — maybe picking out the minimum or maximum OID. However, more than an hour of googling has not yielded any solutions.

Does anybody have this trick in their toolbelt?

Answer 1

I like Charlie’s answer best, but i’m not sure if Postges has a TOP/LIMIT function, so here’s an alternate solution that doesn’t need one (but does assume sub_table has a primary key called ‘id’):

SELECT *  FROM main_table m LEFT OUTER JOIN sub_table s  ON s.main_table_id = m.id WHERE s.id IS NULL OR s.id IN (    SELECT MAX(id) FROM sub_table GROUP BY main_table_id )