--Column ID of table A is primary key:

CREATE TABLE A (
	ID	int4 not null
);
CREATE UNIQUE INDEX AI ON A (ID);

--Columns REFB of table B and REFC of C are foreign keys referenting ID of A:

CREATE TABLE B (
	REFB	int4
);
CREATE INDEX BI ON B (REFB);

CREATE TABLE C (
	REFC	int4
);
CREATE INDEX CI ON C (REFC);

--Trigger for table A:

CREATE TRIGGER AT BEFORE DELETE OR UPDATE ON A FOR EACH ROW
EXECUTE PROCEDURE 
check_foreign_key (2, 'cascade', 'ID', 'B', 'REFB', 'C', 'REFC');
/*
2	- means that check must be performed for foreign keys of 2 tables.
cascade	- defines that corresponding keys must be deleted. 
ID	- name of primary key column in triggered table (A). You may
	  use as many columns as you need.
B	- name of (first) table with foreign keys.
REFB	- name of foreign key column in this table. You may use as many
	  columns as you need, but number of key columns in referenced
	  table (A) must be the same.
C	- name of second table with foreign keys.
REFC	- name of foreign key column in this table.
*/

--Trigger for table B:

CREATE TRIGGER BT BEFORE INSERT OR UPDATE ON B FOR EACH ROW
EXECUTE PROCEDURE 
check_primary_key ('REFB', 'A', 'ID');

/*
REFB	- name of foreign key column in triggered (B) table. You may use as 
	  many columns as you need, but number of key columns in referenced
	  table must be the same.
A	- referenced table name.
ID	- name of primary key column in referenced table.
*/

--Trigger for table C:

CREATE TRIGGER CT BEFORE INSERT OR UPDATE ON C FOR EACH ROW
EXECUTE PROCEDURE 
check_primary_key ('REFC', 'A', 'ID');

-- Now try

INSERT INTO A VALUES (10);
INSERT INTO A VALUES (20);
INSERT INTO A VALUES (30);
INSERT INTO A VALUES (40);
INSERT INTO A VALUES (50);

INSERT INTO B VALUES (1);	-- invalid reference
INSERT INTO B VALUES (10);
INSERT INTO B VALUES (30);
INSERT INTO B VALUES (30);

INSERT INTO C VALUES (11);	-- invalid reference
INSERT INTO C VALUES (20);
INSERT INTO C VALUES (20);
INSERT INTO C VALUES (30);

DELETE FROM A WHERE ID = 10;
DELETE FROM A WHERE ID = 20;
DELETE FROM A WHERE ID = 30;

SELECT * FROM A;
SELECT * FROM B;
SELECT * FROM C;