Django migration sql for conditional triggers - mysql

Question

I want to create a trigger that only inserts into table when the condition is met.
I've tried using various combinations of IF/BEGIN/END and WHERE, but Django returns me an SQL syntax error each time.
Here, type_user_id refers to the person who triggers the like, and user_id refers to the person who receives the notification. It should only trigger when they are not the same person.
operations = [
migrations.RunSQL(
"DROP TRIGGER like_notification",
"""
CREATE TRIGGER like_notification AFTER INSERT ON user_likes_article
FOR EACH ROW
IF (select user_id from channel WHERE channel_id IN
(SELECT channel_id FROM article WHERE article_id = NEW.article_id)) <> NEW.user_id
BEGIN
INSERT INTO notification (type, type_id, article_id, is_read, type_user_id, user_id)
VALUES ('like', NEW.article_id, NEW.article_id, false, NEW.user_id,
(SELECT user_id from channel WHERE channel_id IN
(SELECT channel_id FROM article WHERE article_id = NEW.article_id)
)
)
END;
"""
)
]
>>>django.db.utils.ProgrammingError: (1064, "You have an error in your SQL syntax;
check the manual that corresponds to your MariaDB server version for the right syntax
to use near 'BEGIN\n INSERT INTO notification (type, type_id, article_id, is_re' at line 5")
and
CREATE TRIGGER like_notification AFTER INSERT ON user_likes_article
FOR EACH ROW
BEGIN
DECLARE #user_same Boolean;
SELECT 1 into #user_same
FROM channel
WHERE channel_id IN (select channel_id FROM article where article_id = NEW.article_id)
AND user_id = NEW.user_id;
IF #user_same <> 1
THEN
INSERT INTO notification (type, type_id, article_id, is_read, type_user_id, user_id,)
VALUES ('like', NEW.article_id, NEW.article_id, false, NEW.user_id, (
SELECT user_id from channel WHERE channel_id IN (
SELECT channel_id FROM article WHERE article_id = NEW.article_id)
)
END IF;
END;
>>>You have an error in your SQL syntax; check the manual that corresponds to your
MariaDB server version for the right syntax to use near '#user_same Boolean;\n SELECT 1 into #user_same\n FRO' at line 4"
and
CREATE TRIGGER like_notification AFTER INSERT ON user_likes_article
FOR EACH ROW
INSERT INTO notification (type, type_id, article_id, is_read, type_user_iduser_id)
VALUES ('like', NEW.article_id, NEW.article_id, false, NEW.user_id,
(
SELECT user_id from channel WHERE channel_id IN
(SELECT channel_id FROM article WHERE article_id = NEW.article_id)
)
WHERE NOT user_id = NEW.user_id;
>>>You have an error in your SQL syntax; check the manual that corresponds to your
MariaDB server version for the right syntax to use near 'WHERE NOT user
I don't really understand why none of them work. Any help would be much appreciated!

Answer 1

#variables are not DECLARED.
Either:
DECLARE user_same BOOLEAN;
SELECT 1 INTO user_same WHERE ...
or
SELECT #user_same := 1 WHERE ...
Better yet, this avoids the need for the variable.
IF (EXISTS SELECT * FROM ...)
Also, don't use the construct IN ( SELECT ... ); it is usually better to use WHERE EXISTS ( SELECT * FROM ... ) or JOIN ... ON ....
(There may be more problems after fixing those.)

Answer 2

Another easy way to do this.
You should add any symbols between ; delimiters for the appropriate converting MySQL syntax to the raw SQL.
CREATE TRIGGER trigger_name BEFORE INSERT ON table
FOR EACH ROW
BEGIN
IF NEW.number <> 'anynumber' AND NEW.number <> 'anynumber'
THEN
SET NEW.number = 'anynumber'; --
END IF; --
END
This example works due to the dash symbols.