Question

SELECT user_id,     SUM(COALESCE(point_points, 0)) AS total_points,     SUM(         CASE             WHEN point_date > '$this_month'             THEN point_points             ELSE 0         END)                AS month_points,     COUNT(DISTINCT c_id)    AS num_comments,     COUNT(DISTINCT rant_id) AS live_submissions FROM users     LEFT JOIN points     ON  users.user_id = points.point_userid     LEFT JOIN comments     ON         (             c_userid = user_id         )     LEFT JOIN rants     ON         (             rant_poster = user_id         AND rant_status = 1         ) WHERE user_id = $id GROUP BY user_id 

Basically live_submissions and num_comments variable display proper results, while the total_points and month_points display a product of month_points/total_points, live_submissions and num_comments. Any idea why this is happening?

Answer 1

This is called a Cartesian Product. When you join the tables together, the default result is every permutation of rows for which the join conditions are true. You use JOIN conditions to limit these permutations.

But since you are joining multiple tables to users, the result includes every permutation of each matching table. For example, each matching row in points is repeated per matching row in comments, and each of these is multiplied again, repeating per matching row in rants.

You can partially compensate for this with COUNT(DISTINCT c_id) as you are doing, but the DISTINCT is necessary only because you have multiple rows per c_id. And it doesn’t work unless you apply it to unique values. This remedy doesn’t work for the SUM() expressions.

Basically, you’re trying to do too many calculations in one query. You need to split it up into separate queries for it to be reliable. And then you can get rid of the DISTINCT modifiers, too.

SELECT u.user_id, SUM(COALESCE(p.point_points, 0)) AS total_points,    SUM( CASE WHEN p.point_date > '$this_month' THEN p.point_points ELSE 0 END ) AS month_points FROM users u LEFT JOIN points p   ON u.user_id = p.point_userid  WHERE u.user_id = $id GROUP BY u.user_id;  SELECT user_id, COUNT(c.c_id) as num_comments,  FROM users u LEFT JOIN comments c   ON (c.c_userid = u.user_id) WHERE u.user_id = $id GROUP BY u.user_id;  SELECT u.user_id, COUNT(r.rant_id) as live_submissions FROM users u LEFT JOIN rants r   ON (r.rant_poster = u.user_id AND r.rant_status = 1) WHERE u.user_id = $id GROUP BY u.user_id; 

You shouldn’t try to do all three of these in a single query.