Question

I have an SQL problem i was wondering if someone could help me out. Below is the schema of the database

Player

• playerid (primary key)
• playerName

PlaysAt

• clubId (primary key)
• playerId (fk to PLAYER.playerid)
• yearsAtClub

My question is how do i select the clubId of the club where player named john and stephen play where john and stephen play at the same club. I have no idea how to get the club id in this case. i have managed to get the join part correct as im able to select the club id of john but cant get it when i specify the both players using WHERE playerName = john AND playerName = stephen.

Answer 1

Use:

  SELECT c.clubid
    FROM PLAYSAT c
    JOIN PLAYER p ON p.playerid = c.playerid
   WHERE p.playername IN ('john', 'stephen')
GROUP BY c.clubid
  HAVING COUNT(DISTINCT p.playername) = 2

The key is that the number of parameters in the IN clause needs to match the COUNT in the HAVING clause — in this case, two.

The DISTINCT helps in case there isn’t a primary key or unique constraint on both the PLAYSAT.clubid and PLAYSAT.playerid columns — two entries for “John” otherwise would be considered valid if for the same clubid value. Otherwise, the DISTINCT can be omitted from the query.