根据行的数值更新

Question

I need to make a point system in MySQL, the points are added based on the position of a certain column. Example: there are 5 players, the player with the highest score in "Conqueror" row gets awarded 5 points, the 2nd player gets awarded 4 points, the 3rd player 3 points etc. This is a stored procedure that I would call when calculating points:

BEGIN
select Player_name into @a from Player where Conqueror = (select max(Conqueror) from Player);
select Player_name into @b from Player where Conqueror = (select max(Conqueror) from Player where Player_name != @a);
select Player_name into @c from Player where Conqueror = (select max(Conqueror) from Player where Player_name != @a and Player_name != @b );
select Player_name into @d from Player where Conqueror = (select max(Conqueror) from Player where Player_name != @a and Player_name !=@b and Player_name !=@c);
select Player_name into @e from Player where Conqueror = (select max(Conqueror) from Player where Player_name != @a and Player_name !=@b and Player_name !=@c and Player_name !=@d );
select count(Conqueror) into @playernum from Player;
update Player set Points = Points + @playernum where Player_name = @a;
update Player set Points = Points + @playernum-1 where Player_name = @b;
update Player set Points = Points + @playernum-2 where Player_name = @c;
update Player set Points = Points + @playernum-3 where Player_name = @d;
update Player set Points = Points + @playernum-4 where Player_name = @e;
END

This code looks terrible but it works fine except when there are players with the same score. In that case an MySQL Error #1172 - Result consisted of more than one row due to two or more Player_name's are being added to a variable. Can anyone please give me an idea on how to solve this?

Answer 1

I haven't had time to play with it and come up with a solution where Player 4 gets 2 points instead of 3, but it's probably just a case of adding an extra bit to the IF statement. But based on this blog post:

http://jimmod.com/blog/2008/09/09/displaying-row-number-rownum-in-mysql/

I would actually get rid of the stored procedure and do the whole thing in one update statement with a subselect. Something like (completely untested!):

update Player set points =
  ( select points from
      ( SELECT @rownum:=@rownum-1 'points', Conqueror FROM Player p, 
          (SELECT @rownum:=6) r 
        order by Conqueror desc
      )
  )

This should, if I've guessed the syntax right, allocate 5 to the highest, 4 to the second, etc except if two players have the same points, one will get 4 and the other will get 3. Check the blog post for an IF statement that will solve this - I'll take a look later when I have more time :)

EDIT

In case the blog ever gets taken down, this is the IF statement I am referring to:

select IF(@score=p.score, @rownum, @rownum:=@rownum+1) rank2, p.*, (@score:=p.score) dummy from player p, 
(SELECT @rownum:=0) x, 
(SELECT @score:=0) y 
order by score desc limit 10;

Answer 2

If you don't need to place users with the same Conqueror on the same place you can use this ONE query :-) not 5.

updated query already allow users to have the same Conqueror values and they will be granted same amount of points according to your comment :

Ideally, lets say that player number 2 and 3 have the same score. Player 2 gets awarded 4 points and player 3 also 4 points but player 4 would get rewarded 2 points. So when there are same scores all the players get awarded the points as if they were the only ones with that score. – Voodoo

SET @rank = 6;
UPDATE Player 
INNER JOIN 
(
  SELECT player_id,
    @rank := @rank-1 as rank,
    CASE WHEN @prev = Conqueror 
      THEN @correction := @correction+1 
      ELSE @correction := 0  
    END as correction,
    @prev := Conqueror
  FROM Player
  ORDER BY Conqueror DESC
  LIMIT 5
) as r
ON Player.player_id = r.player_id
SET Points = Points + r.rank + r.correction;

And if you need to place players with the same Conqueror result on the same rank, just give me more rules how to handle such situations.

EDIT - EXPLANATION

Cauze you commented you don't understand what is going on in my query, I'll try to explain a little.

So @prev var holds the value of previous top player executed.

For example for the player who has 2nd rank it will hold the Conqueror value of the 1st player.

We need it to compare if 2nd place player has the same score as 1st to give him same amount of points as you asked in your comment.

r.rank and r.correction holds the RANK (points/place from 5 to 1) for the player that you want to add to Points field. And correction is stands for correct those value if players have same Conqueror and we need to give the same amount of points even if rank are 2 and 3 for example but points added will be the same rank=4 + correction=0 for 2nd player and rank=3 + correction=1 for 3rd player