Combine two SELECT queries in PostgreSQL

Question

I would like to combine two select queries with UNION .
How can I use the result from the first SELECT in the second SELECT ?

(SELECT carto_id_key FROM table1
    WHERE tag_id = 16)
UNION 
(SELECT * FROM table2
    WHERE carto_id_key = <the carto_id result from above> )

Answer 1

Use a CTE to reuse the result from a subquery in more than one SELECT .
You need PostgreSQL 8.4+ for that:

WITH x AS (SELECT carto_id_key FROM table1 WHERE tag_id = 16)

SELECT carto_id_key
FROM   x

UNION ALL
SELECT t2.some_other_id_key
FROM   x
JOIN   table2 t2 ON t2.carto_id_key = x.carto_id_key

You most probably want UNION ALL instead of UNION . Doesn't exclude duplicates and is faster this way.