Question

To clarify – there will be up to 4 distinct players in the table, referenced by their ids. I want to enforce this, i.e. no two ids are the same.

Also, because it’s up to 4 players, some may be null. Is potentially having many nulls like this a bad idea? I don’t think it does, but just to be sure, does null == null return false?

...
CHECK (playerid1 <> playerid2)
  AND (playerid1 <> playerid3)
  AND (playerid1 <> playerid4)
  AND (playerid2 <> playerid3)
  AND (playerid2 <> playerid4)
  AND (playerid3 <> playerid4)
...

Thank you!

Answer 1

I’d use a separate table for this:

 create table game_players (
      game_id number, player_id number, player_position number
    );

Then you can add

• a UNIQUE constraint for game_id+player id (to enforce a player doesn’t occur more than once per game)
• a UNIQUE constraint for game_id+player_position (to ensure a position is occupied by at most one player)
• a CHECK constraint for player_position to enforce it only has the values 1, 2, 3 or 4

This should achieve what you want in a somewhat cleaner (IMHO) way.