UPDATE JOIN tables, mathematical operation happens only on single (last?) row - mysql

Question

In my application I keep track of users' total unread notifications together with other user data in user table. Notifications are in separate table, notifications of similar nature are stacked into a single row, each row tracks count and last timestamp. Now I want to implement a function of marking all notifications as read until a particular timestamp. I made this query using mathematical subtraction between two columns from two different tables:
UPDATE
core_notification n
LEFT JOIN
core_user u
ON
n.notification__user_id = u.user__id
SET
n.notification__if_read = 1,
u.user__notification_unread_count = u.user__notification_unread_count - n.notification__main_count
WHERE
n.notification__timestamp <= 123456 AND
n.notification__if_read = 0 AND
n.notification__user_id = 123;
Problem: lets say in user table "user__notification_unread_count" has a value of 4 and there are two notifications which both have a "notification__main_count" value of 2. My goal after running this query and updating both notifications as read is to have "user__notification_unread_count" value of 0 (4 - 2 - 2 = 0). However the result is always 2. I tried various join methods but it seems that it is just how databases work when updating multiple rows (each updated row overwrites previous update completely, but that is only my assumption).
Is there a way I could achieve the result I want with a single query?

Answer 1

This is a situation where you are probably better off using a trigger. But . . . you can do it in a single update, just with more work:
UPDATE core_notification n LEFT JOIN
core_user u
ON n.notification__user_id = u.user__id LEFT JOIN
(SELECT n2.notification__user_id,
SUM(n2.notification__main_count) as total_notification__main_count
FROM core_notification n2
WHERE n2.notification__timestamp <= 123456 AND
n2.notification__if_read = 0 AND
n2.notification__user_id = 123;
GROUP BY n2.notification__user_id
) n2
ON n2.notification__user_id = n2.notification__user_id
SET n.notification__if_read = 1,
u.user__notification_unread_count = u.user__notification_unread_count - n2.total_notification__main_count
WHERE n.notification__timestamp <= 123456 AND
n.notification__if_read = 0 AND
n.notification__user_id = 123;
The n2 derived table separately calculates the total that needs to be subtracted.