Question

I am trying to delete from multiple tables. Here’s what my tables look like

    A_has_B ---- B ---- C_has_B
(many to many)        (many to many)

I am trying to delete all rows from A_has_B, B and C_has_B given the ID of a record in B. I am using MySQL with the innodb storage engine with foreign keys defined for A_has_B and C_has_B referencing the IDs in B.

I am trying to perform my delete like so:

DELETE A_has_B.*, C_has_B.*, B.*

FROM
A

join
B
on (B.B_id = A.B_id)

join
C
on (C.B_id = B.B_id)

where B.B_id IN(1,2, 4);

The problem is that when I execute the query, mysql complains:

Error Code: 1451. Cannot delete or update a parent row: a foreign key constraint fails (`db`.`C`, CONSTRAINT `fk_C` FOREIGN KEY (`B_id`) REFERENCES `B` (`B_id`) ON DELETE NO ACTION ON UPDATE NO)

How can I go about fixing this?

Answer 1

The simplest way would be to delete from each table individually:

-- Remove all connections from A which reference
-- the B-rows you want to remove
DELETE FROM A_has_B
WHERE B_id IN (1,2,4);

-- Remove all connections from C which reference
-- the B-rows you want to remove
DELETE FROM C_has_B
WHERE B_id IN (1,2,4);

-- Finally remove the B-rows
DELETE FROM B
WHERE B_id IN (1,2,4);

MySQL also allows you to delete from multiple tables in one statement. But there is no way to control the order of the deletions. From the manual:

If you use a multiple-table DELETE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, you should delete from a single table and rely on the ON DELETE capabilities that InnoDB provides to cause the other tables to be modified accordingly.