Syntax error in converting SQL Server trigger to MySQL - mysql

Question

I have to convert SQLServer trigger to MySQL. I was trying to recreate it in MySQL but the thing is that I get syntax error every time I try different combinations and I don't know what's not working and why.
I'm creating cinema database and the following trigger is started when an entry is removed from the Hall table. Suppose a hall is being renovated and we have to move the performances in that hall to other locations. In this case, they are moved to a hall with an index 1 smaller than the distant hall.
Here is SQLServer query:
CREATE TRIGGER tDeleteHall ON Hall
FOR DELETE
AS
BEGIN
DECLARE #id int
SELECT #id = id_hall FROM deleted
UPDATE Spectacle set id_hall = #id - 1
END
DELETE FROM Hall WHERE id_hall = 3;
MySQL code
CREATE TRIGGER tDeleteHall BEFORE DELETE ON Hall
DECLARE #id int
SELECT #id = id_hall FROM deleted
UPDATE Spectacle set id_hall = #id - 1
END
DELETE FROM Spectacle WHERE id_hall = 3;
The error I'm getting:
ERROR: 1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DECLARE #id int
SELECT #id = id_sala FROM deleted
UPDATE Spectacle set id_hall = ' at line 2

Answer 1

It's not totally clear what this trigger is supposed to do. But here's a MySQL trigger that I think does the same thing.
CREATE TRIGGER tDeleteHall BEFORE DELETE ON Hall
FOR EACH ROW
BEGIN
UPDATE Spectacle SET id_hall = OLD.id_hall - 1;
END
MySQL does not have the deleted pseudo-table. Instead, it uses OLD.<column> to refer to the deleted row, or in INSERT and UPDATE triggers, use the NEW.<column> to refer to the new row version.
MySQL's triggers are always row triggers, not statement triggers as in SQL Server. Nevertheless, MySQL triggers require the FOR EACH ROW syntax. Perhaps someday they will also support statement triggers.
You don't need to DECLARE a local variable for the example you show. But if you do use a local variable, know that unlike SQL Server, local variables in MySQL routines must not use the # sigil.
MySQL has two types of variables:
Local variables, which do not use the # sigil. These are created with DECLARE, and assigned a data type. They are local to the code block they are declared in.
User-defined session variables, which do use the # sigil. You can use these inside triggers, but they are not local. In other words, the value you set will remain set in your session after the trigger is done. These variables are created when you set them, so you don't declare them with DECLARE. They don't have a data type.
Statements inside the code block must be terminated with ;. This creates an ambiguity because the CREATE TRIGGER statement itself needs to be terminated. But if you run this in a client that assumes that ; terminates the CREATE TRIGGER, and there are ; characters in the body of the trigger, it becomes confused. The solution is to use DELIMITER to change the terminator of the CREATE TRIGGER, then you can use ; inside the body. You should read https://dev.mysql.com/doc/refman/8.0/en/stored-programs-defining.html
On the other hand, if you execute CREATE TRIGGER using a query interface that only processes the input one statement at a time anyway, there's no ambiguity, so you don't need to change the DELIMITER. Examples would be most query APIs.
Stored routines in MySQL are quite different from SQL Server. You need to make an effort to read the documentation and study examples. It's not an efficient use of your time or ours to try to learn complex syntax via Stack Overflow.