SQL using the result of a UNION query in another query

Question

How can I complete this query?

Right now, the query I have working is this, but it is not producing the right data.

SELECT date, coalesce(count,0) AS count 
FROM 
    generate_series(
        '2014-12-13 12:00:00'::timestamp, 
        '2015-01-06 11:00:00'::timestamp, 
        '1 hour'::interval
    ) AS date 
    LEFT OUTER JOIN (
        SELECT 
            date_trunc('day', TABLE1.created_at) as day, 
            count(DISTINCT TABLE1.user) as count 
        FROM TABLE1 
        WHERE org_id = 1 
        GROUP BY day
    ) results ON (date = results.day);

Instead of TABLE1, I need to feed the query with data from another query which looks like this:

SELECT TABLE2.user_a as userid, TABLE2.created_at as createdat from TABLE2 
UNION ALL 
SELECT TABLE3.user_b as userid, TABLE3.created_at as createdat from TABLE3 
UNION ALL 
SELECT TABLE4.sender as userid, TABLE4.created_at as createdat from TABLE4;

How do I do this?

Answer 1

Any part of a select query that receives a table (eg, a from clause, a join clause, etc) can receive a query surrounded in parenthesis - this is called a subquery. Note that in Postgres this subquery must be given an alias (ie, a name that it can be referenced by). So in your case:

SELECT date, coalesce(count,0) AS count 
FROM 
    generate_series(
        '2014-12-13 12:00:00'::timestamp, 
        '2015-01-06 11:00:00'::timestamp, 
        '1 hour'::interval
    ) AS date 
    LEFT OUTER JOIN (
        SELECT 
            date_trunc('day', subquery.created_at) as day, 
            count(DISTINCT subquery.user) as count 
              -- Subquery here:
        FROM (SELECT TABLE2.user_a as userid, TABLE2.created_at as createdat from TABLE2 
              UNION ALL 
              SELECT TABLE3.user_b as userid, TABLE3.created_at as createdat from TABLE3 
              UNION ALL 
              SELECT TABLE4.sender as userid, TABLE4.created_at as createdat from TABLE4) 
             subquery 
        WHERE org_id = 1 
        GROUP BY day
    ) results ON (date = results.day);