Question

I have a two tables: users and missionInfo. I need to select classes only when all of the users who are in that class completed all missions.

users table:
userID  classID
1       1
2       1
3       1
4       2
5       2

missionInfo table:
userID  missionID   
1       1
1       2
1       3
2       1
2       2
2       3
3       1
3       2
3       3
4       1
5       1
5       2

So, I should get back classID 1 because users 1 thru 3 all completed all missions 1 thru 3 and are all part of the same class.

Help?

Answer 1

SELECT classID FROM (
  SELECT   classID
  FROM     users JOIN missionInfo USING (userID)
  GROUP BY classID, missionID
  HAVING   COUNT(DISTINCT userID) = (
    SELECT COUNT(*) FROM users u WHERE u.classID = users.classID
  )
) AS t
GROUP BY classID
HAVING COUNT(*) = @total_number_of_missions

See it on sqlfiddle.

The DISTINCT can be omitted if (userID,missionID) will only appear once in the missionInfo table.

The inner query returns classID for every mission that has been completed by all that class’s users; the outer one restricts the results to only those classes who have completed the specified number of missions.