MySQL Trigger to update all entries with a many-many relationship - mysql

Question

I am trying to make a MySQL trigger that will update all my users scores if a levels reward is updated. I have got as far as working out the difference between the old and new reward but am stuck on how to update every users score who has completed a level of that. Below is a simplified table structure.
Users
+---------+-------+
| user_id | score |
+---------+-------+
Users_levels
+---------+----------+
| user_id | level_id |
+---------+----------+
Levels
+----------+---------+
| level_id | tier_id |
+----------+---------+
Tier_reward
+---------+----------+
| tier_id | reward |
+---------+----------+
This is how far I have got so far:
CREATE TRIGGER update_level_reward AFTER UPDATE ON tier_reward FOR EACH ROW
BEGIN
DECLARE REWARD INT;
SET REWARD = OLD.reward - NEW.reward;
-- UPDATE users SET score = score - REWARD WHERE user_id = ;
END$$
I have tried something along the lines of:
UPDATE users
INNER JOIN users_levels
ON users.user_id = users_levels.user_id
INNER JOIN levels
ON users_levels.level_id = levels.level_id
SET score = score - REWARD
WHERE levels.tier = OLD.tier;
However this only reduces the score once even if a user has completed more than one level from that tier.
Example
Users
+---------+-------+
| user_id | score |
+---------+-------+
| 1 | 400 |
| 2 | 700 |
+---------+-------+
Users_levels
+---------+----------+
| user_id | level_id |
+---------+----------+
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 2 | 3 |
+---------+----------+
Levels
+----------+---------+
| level_id | tier_id |
+----------+---------+
| 1 | 1 |
| 2 | 1 |
| 3 | 2 |
+----------+---------+
Tier_reward
+---------+----------+
| tier_id | reward |
+---------+----------+
| 1 | 200 |
| 2 | 500 |
+---------+----------+
Now if the reward for tier_id 1 is reduced to 100. User 1 should now have a score of 200 as they have completed two levels of that tier. User 2 should loose only 100 points as they have only completed one level of that tier.
Users
+---------+-------+
| user_id | score |
+---------+-------+
| 1 | 200 |
| 2 | 600 |
+---------+-------+
Tier_reward
+---------+----------+
| tier_id | reward |
+---------+----------+
| 1 | 100 |
| 2 | 500 |
+---------+----------+

Answer 1

Sorry about the delay !!
Here is what you can do, you need to first get the count per tier per user. Then use cursor to set these values and finally update the users table.
delimiter //
create trigger update_level_reward after update on tier_reward
for each row
begin
DECLARE done INT DEFAULT FALSE;
DECLARE tier_count int;
DECLARE user_id_fetch int;
DECLARE reward INT;
DECLARE reward_recal int ;
DECLARE cur CURSOR FOR
select
ul.user_id, count(*) as total
from users_levels ul
join levels l on l.level_id = ul.level_id
where l.tier_id = new.tier_id
group by ul.user_id;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
SET reward = OLD.reward - NEW.reward;
OPEN cur;
update_loop: LOOP
FETCH cur INTO user_id_fetch,tier_count;
IF done THEN
LEAVE update_loop;
END IF;
set reward_recal = tier_count*reward ;
update users
SET score = score - reward_recal
where user_id = user_id_fetch ;
END LOOP;
CLOSE cur;
end ; //
delimiter ;
Here what I did in mysql
create table users (user_id int,score int);
insert into users values (1,400),(2,700);
create table users_levels(user_id int,level_id int);
insert into users_levels values
(1,1),(1,2),(2,1),(2,3);
create table levels (level_id int,tier_id int);
insert into levels values
(1,1),(2,1),(3,2);
create table tier_reward(tier_id int,reward int);
insert into tier_reward values
(1,200),(2,500);
mysql> update tier_reward set reward = 100 where tier_id = 1 ;
Query OK, 1 row affected (0.15 sec)
mysql> select * from users ;
+---------+-------+
| user_id | score |
+---------+-------+
| 1 | 200 |
| 2 | 600 |
+---------+-------+
2 rows in set (0.00 sec)