Question

I had this question at a job interview yesterday and although it seemed pretty straight fwd I couldn’t figure it out and it’s kept me awake all night thinking about it.

The system records data about a scrabble league there is a members table, a games table and a joining member_games table.

members: member_id, name : pk(member_id)
member_games: game_id, member_id, score : pk(game_id, member_id)
games: game_id, location, date : pk(game_id)

members
1, mick
2, keith
3, charlie

member_games
1, 1, 50
1, 2, 60
2, 1, 45
2, 3, 105
3, 1, 30
3, 3, 120

game
1, london, 2012-12-01
2, richmond, 2012-12-02
3, leeds, 2012-12-03

How do you formulate an SQL query to find out the number of wins for the member_id = 1?

Answer 1

The following query will give you the number of wins for member_id 1.

select count(*) as number_of_wins
    from member_games as mg1
    where member_id = 1
    and not exists (select 1
        from member_games as mg2
        where mg2.game_id = mg1.game_id
        and mg2.member_id <> mg1.member_id
        and mg2.score >= mg1.score);

The following query will give you the list of members with their respective number of wins.

select m.*,
    (select count(*) from member_games as mg1
        where member_id = m.member_id
        and not exists (select 1
            from member_games as mg2
            where mg2.game_id = mg1.game_id
            and mg2.member_id <> mg1.member_id
            and mg2.score >= mg1.score)
    ) as number_of_wins
    from members as m;

These queries do not consider ties for first place as wins.

I created a MySQL SQL Fiddle demo and a SQL Server SQL Fiddle demo.