如何从MySQL中的联接表中获取值的总和？

Question

I have two tables with values similar to the data below.

Scores Table      Posts Table
postId score      id   user
     1     1       1   me
     2    -1       2   you
     1     1       3   me
     2     2
     3     0

From these two tables, I'd like to calculate the score for a given user with a stored procedure that I'd call like getScoreForUser(user).

The results in this exact case would return
CALL getScoreForUser(me) = 2
CALL getScoreForUser(you) = 1

I've tried some pretty stupid things so far:

select score.scores FROM scores INNER JOIN posts ON (scores.postId=posts.id);

which is obviously not even close. Is a join even appropriate in this case?

I'm happy to add them up myself if it's easier (more efficient in Node.js than MySQL for example), though I understand that there are some arithmetic operations available in MySQL.

Answer 1

I think this should do the trick, assuming I've understood your database schema correctly:

SELECT id, SUM(scores.score) AS totalScore
FROM posts
JOIN scores
ON scores.postId = posts.id;

From there, you should be able to use totalScore as expected.

The relevant bit here is the MySQL SUM() function , which we use to add up all the scores.

Answer 2

Your query to group all the items scores based on id will be:

SELECT
  a.id,
  SUM(b.scores) as score
FROM
  posts as a
LEFT JOIN
  Scores as b
ON
  a.id = b.postId
GROUP BY
  a.id

You can then take that and add a where statement in for specific id's in your stored procedure

Answer 3

Ultimately, this is what I used:

CREATE PROCEDURE getScoreForUser(IN inUser VARCHAR(255))
BEGIN
    SELECT id, SUM(scores.score) AS totalScore
    FROM posts
    JOIN scores
    ON scores.postId = posts.id
    WHERE posts.user = inUser;
END

I'm using this in a Node application where we originally thought that stored procedures were the magical cure to sql injection which is why we're still using them, even though we've learned that's not quite true.

Answer 4

Using a stored procedure for this seems a bit of overkill for such a straightforward query. If you already know the integer id then you don't need the JOIN. However, assuming that you want to pass in the user's name the procedure would look like this:

DELIMITER //
DROP PROCEDURE IF EXISTS getScoreForUser //
CREATE PROCEDURE getScoreForUser(IN v_user VARCHAR(32))
BEGIN
  SELECT SUM(score) AS totalScore
  FROM posts p
  JOIN scores s
  ON (s.postId = p.id)
  WHERE p.user = v_user;
END //

DELIMITER ;

CALL getScoreForUser('me');

Also, note you have to use quotes around the name in the CALL to the procedure.