Question

I have an users table where I store user info along with a reputation score like Stackoverflow has.

My question is how to update multiple users’ reputations with a single MySQL query.

Here’s my users table

user_id  reputation
  1          11
  2         202
  3        3003
  4         444
  5         555

Let’s say user_id=1 downvotes user_id=4 and user_id=5. In this scenario user_id=1’s reputation loses 2 points and user_id=4,5 both lose 10 points.

The users table would then look like this:

user_id  reputation
  1           9 //lost 2 points
  2         202
  3        3003
  4         434 //lost 10 points
  5         545 //lost 10 points

I know the MySQL query shown below works but I’m wondering a generalizable solution in the case that the number of users to UPDATE is variable and depends on the voting.

UPDATE `users`
 SET `reputation` = CASE
 WHEN `user_id` = '1' THEN reputation - 2
 WHEN `user_id` = '4' THEN reputation - 10
 WHEN `user_id` = '5' THEN reputation - 10
 ELSE `reputation` END

Answer 1

You can replace the second WHEN with an IN() clause. It still requires some dynamic query building in your application code, but if your API allows direct binding of arrays into an IN() clause, this is much easier.

UPDATE `users`
 SET `reputation` = CASE
 WHEN `user_id` = '1' THEN reputation - 2
 WHEN `user_id` IN ('2','3','4') THEN reputation - 10
 ELSE `reputation` END

I will admit I find it a bit awkward to force all of this into one query though. Unless performance is a real issue, I would rather see this as two queries wrapped in a transaction – one to subtract -2 from user_id=1 and one to subtract the -10 users.

/* Only on InnoDB tables... */
START TRANSACTION;
Update `users` SET `reputation` = `reputation` - 2 WHERE `user_id` = 1;
Update `users` SET `reputation` = `reputation` - 10 WHERE `user_id` IN (2,3,4);
COMMIT;

Documentation on MySQL transaction handling…