Question

I have a little probleme using Triggers in MySQL.

Suppose we have 2 tables:

• TableA
• TableB

And 2 Triggers:

• TriggerA: fires when deleting on TableA and updates TableB
• TriggerB: fires when deleting on TableB and deletes in TableA

The problem is that when I delete some rows in TableB, TriggerB fires and deletes some elements in TableA, then TriggerA fires and tries to update TableB.

It fails because TriggerA tries to update some rows in TableB that are being deleted.

How can I avoid this circular dependencies?

None of those two Triggers are useless, so I don’t know what am I supposed to do to solve this.

Answer 1

Try to use variable.

First trigger:

CREATE TRIGGER trigger1
  BEFORE DELETE
  ON table1
  FOR EACH ROW
BEGIN
  IF @deleting IS NULL THEN
    SET @deleting = 1;
    DELETE FROM table2 WHERE id = OLD.id;
    SET @deleting = NULL;
  END IF;
END

Second trigger:

CREATE TRIGGER trigger2
  BEFORE DELETE
  ON table2
  FOR EACH ROW
BEGIN
  IF @deleting IS NULL THEN
    SET @deleting = 1;
    DELETE FROM table1 WHERE id = OLD.id;
    SET @deleting = NULL;
  END IF;
END

And additional AFTER DELETE triggers:

CREATE TRIGGER trigger3
  AFTER DELETE
  ON table1
  FOR EACH ROW
BEGIN
  SET @deleting = NULL;
END

CREATE TRIGGER trigger4
  AFTER DELETE
  ON table2
  FOR EACH ROW
BEGIN
  SET @deleting = NULL;
END