CASCADE DELETE on two foreign key constraints

Question

I have the following example:

Table A
   -some_id

Table B
   -another_id

Table C
   -some_id_fk
   -another_id_fk

I want to cascade a row on

Answer 1

I suggest two foreign key constraints with ON DELETE SET NULL and a trigger that takes care of the rest

Tables:

CREATE TABLE a (a_id serial PRIMARY KEY, a text NOT NULL);
CREATE TABLE b (b_id serial PRIMARY KEY, b text NOT NULL);

CREATE TABLE ab (
  ab_id serial PRIMARY KEY
, a_id int REFERENCES a ON DELETE SET NULL
, b_id int REFERENCES b ON DELETE SET NULL
, UNIQUE (a_id, b_id)
);

Trigger:

CREATE OR REPLACE FUNCTION trg_ab_upbef_nulldel()
  RETURNS trigger AS
$func$
BEGIN
DELETE FROM ab WHERE ab_id = NEW.ab_id;
RETURN NULL;
END
$func$ LANGUAGE plpgsql;

CREATE TRIGGER upbef_nulldel
BEFORE UPDATE OF a_id, b_id ON ab
FOR EACH ROW
WHEN (NEW.a_id IS NULL AND
      NEW.b_id IS NULL)
EXECUTE PROCEDURE trg_ab_upbef_nulldel();

SQL Fiddle.

• Be sure to have a surrogate PK column for the connecting table. (a_id, b_id) cannot be the PK anyway, because that would disallow NULL in both. Add a UNIQUE constraint instead, which allows NULL values.

• The trigger is optimized for performance and only kicks in when one of the two FK columns is updated, and only when that results in both being NULL.

• The trigger function is trivial: deletes the row and returns NULL to cancel the now void cascaded UPDATE.