Question

I have the correlated subquery below. Each id (147,148,149) in table1 has many records in table2. The id 148 however has no records that match the time condition in the statement. Therefore it is not included in the output. I want it to be included with a 0 for the count column. What needs to be changed?

SELECT b.fkid, COUNT(DISTINCT username)
FROM table2 AS b
WHERE
b.fkid IN ( 147,148,149 )
AND time > (SELECT SUBTIME(a.endTime, SEC_TO_TIME( 60*60 )) FROM table1 AS a WHERE a.id = b.fkid)
GROUP BY b.fkid

This statement returns:

b.fkid   COUNT(DISTINCT username)
147      41
149      26

I want it to return:

b.fkid   COUNT(DISTINCT username)
147      41
148       0
149      26

Okay I got the solution. It is a modified version of rexem’s answer:

SELECT t.fkid,
   IFNULL(nu.num_users, 0)
FROM TABLE_2 t
LEFT JOIN (SELECT t.fkid,
                  COUNT(DISTINCT t.username) 'num_users'
           FROM TABLE_2 t
           JOIN TABLE_1 a ON a.id = t.fkid
                          AND SUBTIME(a.endTime, SEC_TO_TIME( 60*60 )) < t.time
           GROUP BY t.fkid) nu ON nu.fkid = t.fkid
WHERE t.fkid IN (147, 148, 149)
GROUP BY t.fkid, nu.num_users

Changes from rexem's answer:
"SEC_TO_TIME( 60*60 )) = t.time" to "SEC_TO_TIME( 60*60 )) < t.time"
Removed "t.time" in GROUP BY clause of subquery

Answer 1

Try:

   SELECT t.fkid,
          IFNULL(nu.num_users, 0)
     FROM TABLE_2 t
LEFT JOIN (SELECT t.fkid,
                  COUNT(DISTINCT t.username) 'num_users'
             FROM TABLE_2 t
             JOIN TABLE_1 a ON a.id = t.fkid
                           AND SUBTIME(a.endTime, SEC_TO_TIME( 60*60 )) = t.time
         GROUP BY t.fkid) nu ON nu.fkid = t.fkid
   WHERE t.fkid IN (147, 148, 149)
GROUP BY t.fkid, nu.num_users