MySQL: Reference current row in delete trigger - mysql

Question

I'm having trouble referencing the current row in an AFTER DELETE trigger in MySQL. Pretend I have the following books table:
+----+------+----------+
| id | name | ordering |
+----+------+----------+
| 1 | It | 3 |
| 2 | Cujo | 1 |
| 3 | Rage | 2 |
+----+------+----------+
I want to create a trigger that will decrement all rows whose ordering value is greater than the ordering value in a row that is deleted. For example, if I do DELETE FROM books WHERE id = 2, I want the resulting table to look like:
+----+------+----------+
| id | name | ordering |
+----+------+----------+
| 1 | It | 2 |
| 3 | Rage | 1 |
+----+------+----------+
I've tried:
DROP TRIGGER IF EXISTS reorder_books_on_delete;
DELIMITER $$
CREATE TRIGGER reorder_books_on_delete
AFTER DELETE ON books
FOR EACH ROW
BEGIN
IF ordering > OLD.ordering
THEN
UPDATE books SET ordering = ordering - 1
WHERE id = id;
END IF;
END$$
DELIMITER ;
But this results in an error when I execute a DELETE on the table:
ERROR 1054 (42S22): Unknown column 'ordering' in 'where clause'
This refers to the if statement, so how do I reference the current row in an ON DELETE trigger? The column definitely does exist.

Answer 1

The reason why it fails is because there is no current row and hence, ordering column doesn't exist, it should be used in WHERE clause, like this:
DROP TRIGGER IF EXISTS reorder_books_on_delete;
DELIMITER $$
CREATE TRIGGER reorder_books_on_delete
AFTER DELETE ON books
FOR EACH ROW
BEGIN
UPDATE books SET ordering = ordering - 1
WHERE ordering > OLD.ordering;
END$$
DELIMITER ;
However, as per MySQL's documentation, you can't do this, it will return the following error:
SQL Error (1442): Can't update table 'books' in stored
function/trigger because it is already used by statement which invoked
this stored function/trigger.
So, you will have to run another UPDATE query after DELETE query in a single transaction to achieve this functionality.

Answer 2

As said by Darshan, you can't do this with a Trigger.... But, Procedure can make if for you !
DROP PROCEDURE IF EXISTS delete_book;
DELIMITER //
CREATE PROCEDURE delete_book(IN pId INT)
BEGIN
set #a = (
SELECT ordering
FROM books
WHERE id = pId
);
UPDATE books SET ordering = ordering - 1
WHERE ordering > #a;
delete from books
WHERE id = pId;
END
//
DELIMITER ;
And you just have, instead of your DELETE FROM books WHERE id = 4 to make a CALL delete_book(4);