Combine two SELECT queries in PostgreSQL

common-table-expression, postgresql, sql, union

Solution

Use a CTE to reuse the result from a subquery in more than one `SELECT`.

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

SELECT carto_id_key
FROM   cte

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

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

Problem

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> ) ```

Original source