Question

I have this calculation/query here:

SELECT  u.username,
        (a.totalCount * 7) +
        (b.totalCount * 3) +
        (c.totalCount * 1) AS totalScore
        FROM users u 
        LEFT JOIN
            (
                SELECT user_id, COUNT(user_id) totalCount
                FROM items 
                GROUP BY user_id
            ) a ON a.user_id= u.user_id
        LEFT JOIN
            (
                SELECT user_id, COUNT(user_id) totalCount
                FROM comments
                GROUP BY user_id
            ) b ON b.user_id= u.user_id
        LEFT JOIN
            (
                SELECT user_id, COUNT(user_id) totalCount
                FROM ratings
                GROUP BY user_id
            ) c ON c.user_id = u.user_id

        ORDER BY totalScore DESC LIMIT 10;

The problem is, if either a,b, or c returns 0, the entire totalScore is 0. I can’t figure out what is going on? I am not multiplying the final tally by 0 I don’t think?

Answer 1

I think it’s rather a null problem (with your LEFT JOIN, this might easily happen).

And NULL + 1 + 2 = NULL

So use the COALESCE (if null then…) operator

so

SELECT  u.username,
        (COALESCE(a.totalCount, 0) * 7) +
        (COALESCE(b.totalCount, 0) * 3) +
        (COALESCE(c.totalCount, 0) * 1) AS totalScore

and a little SqlFiddle