Set variable if null in trigger - mysql

Question

I have a table for categories with "order" column for ordering the categories.
I'm trying to set order incrementally in trigger before insert:
CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories` FOR EACH ROW BEGIN
DECLARE max_order INT DEFAULT 0;
SELECT MAX(categories.order) INTO max_order FROM categories;
SET NEW.order = max_order + 1;
END;
But if there are no records in the db, order column is being set to NULL.
I've modified the trigger code:
CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories` FOR EACH ROW BEGIN
DECLARE max_order INT DEFAULT 0;
SELECT MAX(categories.order) INTO max_order FROM categories;
IF (ISNULL(#max_order)) THEN BEGIN
SET max_order = 0;
END;
SET NEW.order = max_order + 1;
END;
And I'm getting the following error:
Error Code: 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 '' at line 9
Line 9 is:
IF (ISNULL(#max_order)) THEN BEGIN
I've tried to remove the "#", but still getting the same error. How to fix it?

Answer 1

You need to change the delimiter. Otherwise the DB thinks your trigger definition ends at the first ; which would be incomplete.
Also remove the BEGIN from your IF statement and add use END IF to end your IF
delimiter |
CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories`
FOR EACH ROW BEGIN
DECLARE max_order INT DEFAULT 0;
SELECT MAX(categories.order) INTO max_order FROM categories;
IF (ISNULL(#max_order)) THEN
SET max_order = 0;
END IF;
SET NEW.order = max_order + 1;
END
|
delimiter ;

Answer 2

Besides delimiter you need an END IF
DELIMITER //
CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories` FOR EACH ROW BEGIN
DECLARE max_order INT DEFAULT 0;
SELECT MAX(categories.order) INTO max_order FROM categories;
IF (ISNULL(#max_order)) THEN
SET max_order = 0;
END IF;
SET NEW.order = max_order + 1;
END; //
DELIMITER ;