Strange contraint help please...

Using Oracle 8i on win NT 4 server(sp6a)

How can I get around this problem? - I have 3 tables with one column each -

A
=

ID number(8)

B
=

ID number(8)

C
=

ID number(8)

containin data

A
=

1
2
3

C
=

4
5
6

B
=

1
2
3
4
5
6

how can I add a referential integrity constraints to table B that ensures the values exist in tables A OR C ?

This code below fails as 4,5,6 are in table B -

ALTER TABLE b ADD CONSTRAINT A_FK

 	FOREIGN KEY (ID) 
  		REFERENCES a (ID) ;