Question

I am building an application where users can sign up and then challenge each other (any other). When user 1 has challenged user 2, the score that exists between them is updated and stored (default score is 1 towards each other user). Mutually, user 2 can challenge user 1 and this updates another score.

The simplest way to represent this information is a n*n matrix (with a blank diagonal bcs you don’t have a “score” against yourself). My question is : how do you store it using MySQL ? I was thinking of a table with 3 columns : challenger, challenged, score, but that would result in a n² size and such an exponential factor seems inappropriate for such a simple request. Is there another, intended way to handle matrixes with MySQL ?

Thanks

Answer 1

Your initial idea seems right.
BTW n² is quadratic, not exponential.

MySQL should deal just fine with this.

CREATE TABLE score (
  id integer PRIMARY KEY AUTOINCREMENT NOT NULL,
  player1_id integer NOT NULL,
  player2_id integer NOT NULL,
  score_p1 integer NOT NULL,
  score_p2 integer NOT NULL,
  chal_date date KEY NOT NULL,
  /*UNIQUE KEY p1p2 (player1_id, player2_id),*/
  FOREIGN KEY p1 (player1_id) REFERENCES player.id 
    ON DELETE CASCADE ON UPDATE CASCADE,
  FOREIGN KEY p2 (player2_id) REFERENCES player.id 
    ON DELETE CASCADE ON UPDATE CASCADE)
 Engine=InnoDB;

Note that you only create a row when an actual challenge has taken place and a score needs to be recorded. So you will have exactly one row per challenge. If the same players can challenge each other more then once you should remove the unique key p1p2.

Usage examples

Now if you want to know how many points a player has scored you do

SELECT ifnull(sum(a.score_p1),0) + ifnull(sum(b.score_p2),0) AS total_score 
FROM player
INNER JOIN score a ON (a.player1_id = player.id) 
INNER JOIN score b ON (b.player2_id = player.id)
WHERE player.id = 587;

If you want to know all the players a player has challenged you do

SELECT score.player1 as opponent FROM score 
WHERE score.player2 = 587
UNION ALL
SELECT score.player2 as opponent FROM score
WHERE score.player1 = 587;

If you do a small trick and add the following trigger, you can force that
player1.id < player2.id

DELIMITER $$

CREATE TRIGGER bi_score_each BEFORE INSERT ON score FOR EACH ROW
BEGIN
  DECLARE temp integer;

  /*Force player1.id to always be smaller than player2.id*/
  IF new.player1_id > new.player2_id THEN BEGIN
    SET temp = new.player1_id;
    SET new.player1_id = new.player2_id;
    SET new.player2_id = temp;
  END; END IF;
END$$

DELIMITER;

Now you can simplify your query to see the cumulative score between two players.

@p1:= 578;
@p2:= 789;

SELECT sum(score.score_p1) as score_p1
  ,sum(score.score_p2) as score_p2
FROM score 
WHERE score.player1_id = if(@p1<@p2,@p1,@p2) 
  AND score.player2.id = if(@p1>@p2,@p1,@p2);

Use the following query to select all players a player has battled with the scores

  SELECT score.player2_id as opponent_id
    ,sum(score.score_p2) as opponent_score
    ,sum(score.score_p1) as player_score
    ,p2.name as opponent_name
    ,p1.name as player_name
  FROM score
  INNER JOIN player p2 ON (p2.id = score.player2_id) 
  INNER JOIN player p1 ON (p1.id = score.player1_id)
  WHERE player1_id = 587
  GROUP BY player2.id
UNION ALL
  SELECT score.player1_id as opponent_id
    ,sum(score.score_p1) as opponent_score
    ,sum(score.score_p2) as player_score
    ,p1.name as opponent_name
    ,p2.name as player_name
  FROM score
  INNER JOIN player p2 ON (p2.id = score.player2_id) 
  INNER JOIN player p1 ON (p1.id = score.player1_id)
  WHERE player2_id = 587
  GROUP BY player1.id

Hopes this gives you an idea of what is possible.