mysql trigger after update column from another table - mysql

Question

I have 2 tables which are below.
I would like to create a trigger for table_one. when changed the username in table_one, update user_id value from the table_two user value.
table_one:
+--------------+----------+
| user_id | username |
+--------------+----------+
| 15 | robin |
| 46 | albert |
+--------------+----------+
table_two:
+--------------+----------+
| id | user |
+--------------+----------+
| 1 | john |
| 2 | jack |
| 3 | robin |
| 4 | kylie |
| 5 | robert |
| 6 | albert |
| 7 | jay |
+--------------+----------+
thanks in advance

Answer 1

Do a BEFORE UPDATE trigger, not an AFTER UPDATE trigger.
(I don't think it's possible in an AFTER UPDATE trigger to modify the row that was updated, that fired the trigger. I could be wrong about that, but I just can't wrap my brain around how that would work.)
This is a demonstration of a BEFORE UPDATE trigger on table_one that assigns a value to the user_id column (of the row being updated) based on the result from a query:
DELIMITER $$
CREATE TRIGGER ...
BEFORE UPDATE ON table_one
BEGIN
# local variable we will temporarily store a value fetched from a query
DECLARE li_new_id BIGINT DEFAULT NULL; # match datatype of table_two.id
# lookup `id` value from `table_two` with a SQL query
SELECT s.id
INTO li_new_id
FROM table_two s
WHERE s.user = NEW.username
ORDER BY s.id
LIMIT 1 ;
# assign the value we fetched to the `user_id` column of the row being updated
SET NEW.user_id := li_new_id ;
END$$
DELIMITER ;
Note that if the query doesn't find a matching row, the local variable will have a NULL value. So the SET statement will assign NULL to to the user_id column. There's no check in the trigger that the value we assign won't violate a constraint (NOT NULL, UNIQUE, FOREIGN KEY).