Query that can get player's name if it is not null - mysql

Question

I am not sure what to call this but basically, my database has games that can be played by 1 to 4 players.
The Games table has 4 foreign keys to the PlayerGames table. GameFirstPlace, GameSecondPlace... etc. They can be null.
If they are not they point to an entry in the PlayerGames table.
The PlayerGames table has, among other things, a foreign key to the players table. The players table has PlayerName.
I want to get the PlayerName for all the participants of a Game where PlayerGame is not null.
That is, if my game looks like:
GameFirstPlace GameSecondPlace GameThirdPlace GameFourthPlace
6 7 NULL NULL
Then PlayerGame id 6 has PlayerID = 7 and PlayerGame id 7 has PlayerID 3
Then Player with id 7 PlayerName = 'Jack' and Player with id 3 PlayerName = 'Mary'
Then my query might return:
GameID FirstPlaceName SecondPlaceName ThirdPlaceName FourthPlaceName
5 'Jack' 'Mary' NULL NULL
What might the select query for something like this look like?

Answer 1

SELECT
Game.GameID
,Player1.Name AS FirstPlaceName
,Player2.Name AS SecondPlaceName
,Player3.Name AS ThirdPlaceName
,Player4.Name AS FourthPlaceName
FROM
Game
LEFT OUTER JOIN PlayerGame as PlayerGame1 ON Game.GameFirstPlace = PlayerGame1.Id
LEFT OUTER JOIN Player AS Player1 ON PlayerGame1.PlayerId = Player1.Id
LEFT OUTER JOIN PlayerGame as PlayerGame2 ON Game.GameSecondPlace = PlayerGame2.Id
LEFT OUTER JOIN Player AS Player2 ON PlayerGame2.PlayerId = Player2.Id
LEFT OUTER JOIN PlayerGame as PlayerGame3 ON Game.GameSecondPlace = PlayerGame3.Id
LEFT OUTER JOIN Player AS Player3 ON PlayerGame3.PlayerId = Player3.Id
LEFT OUTER JOIN PlayerGame as PlayerGame4 ON Game.GameFirstPlace = PlayerGame4.Id
LEFT OUTER JOIN Player AS Player4 ON PlayerGame4.PlayerId = Player4.Id

Answer 2

You're going to have to duplicate all of the join logic four ways.
select
g.GameID,
p1.PlayerName as FirstPlaceName,
p2.PlayerName as SecondPlaceName,
p3.PlayerName as ThirdPlaceName,
p4.PlayerName as FourthPlaceName
from
Games g
left outer join PlayerGames pg1 on pg1.PlayerID = g.GameFirstPlace
left outer join Players p1 on p1.PlayerID = pg1.PlayerID
left outer join PlayerGames pg2 on pg2.PlayerID = g.GameSecondPlace
left outer join Players p2 on p2.PlayerID = pg2.PlayerID
left outer join PlayerGames pg3 on pg3.PlayerID = g.GameThirdPlace
left outer join Players p3 on p3.PlayerID = pg3.PlayerID
left outer join PlayerGames pg4 on pg4.PlayerID = g.GameFourthPlace
left outer join Players p4 on p4.PlayerID = pg4.PlayerID
I'm guessing that your PlayerGame table has a GameID which you could take advantage of to simplify the join logic. The output gets a little more complicated in return but the query will probably perform better.
SELECT
g.GameID,
min(case when p.PlayerID = g.GameFirstPlace then p.PlayerName end) AS FirstPlaceName,
min(case when p.PlayerID = g.GameSecondPlace then p.PlayerName end) AS SecondPlaceName,
min(case when p.PlayerID = g.GameThirdPlace then p.PlayerName end) AS ThirdPlaceName,
min(case when p.PlayerID = g.GameFourthPlace then p.PlayerName end) AS FourthPlaceName
FROM
Games g
inner join PlayerGames pg on pg.GameID = g.GameID
inner join Players p on p.PlayerID = pg.PlayerID
GROUP BY
g.GameID

Answer 3

SELECT GameID,
P1.PlayerName FirstPlaceName,
P2.PlayerName SecondPlaceName,
P3.PlayerName ThirdPlaceName,
P4.PlayerName FourthPlaceName
FROM Games G
LEFT JOIN PlayerGames P1
ON G.GameFirstPlace = P1.PlayerID
LEFT JOIN PlayerGames P2
ON G.GameSecondPlace = P2.PlayerID
LEFT JOIN PlayerGames P3
ON G.GameThirdPlace = P3.PlayerID
LEFT JOIN PlayerGames P4
ON G.GameFourthPlace = P4.PlayerID