How to declare a foreign key with an OR condition using Oracle?

Question

I have a table (A) whose primary key is either a foreign key to table (B) or table (C).

create table A (
  akey number, 
  txt varchar2(10)
);

create table B (
  bkey number, 
  txt varchar2(10)
);

create table C (
  ckey number, 
  txt varchar2(10)
);

What I want is something like:

alter table A add constraint BorCkey foreign key (akey) references B(bkey)` or C(ckey);

Is this possible?

Answer 1

No, that sort of thing is not possible in Oracle.

Your options generally are

• Create two different columns (bkey and ckey) in A where bkey references B.bkey and ckey references C.ckey and create a constraint that ensures that only one is non-NULL at any point in time.
• Create some sort of "combined B & C" entity that B & C have foreign keys to and make the foreign key in A reference the key of this combination entity.

If you want a constraint that ensures that exactly one of two columns is NULL and one is NOT NULL for any row

create table one_key( 
  col1 number, 
  col2 number, 
  check( nvl2(col1,1,0) + nvl2(col2,1,0) = 1 ) 
)

Answer 2

A foreign key constraint is to one foreign table.
That means you'd need to use two ALTER TABLE statements in this situation to setup foreign keys to reference the two tables. There's no opportunity in there to specify an OR in the relationship -- the value in A.akey would have to exist in both B.bkey and C.ckey at the same time. For example, if B.bkey has a value of NULL, but C.ckey does not -- then A.akey can never have a value of NULL. Foreign keys are deferrable in Oracle, but the behavior described is what you will encounter if both foreign keys are enabled at the same time -- you won't be able to enable a constraint if all the values don't satisfy the relationship.

You need to review your needs for how to simplify the relationship so it doesn't need two tables to make this work.

Answer 3

Sounds like you have some form of subtype/supertype relationship going on. A typical example is 'PERSON' which may be either a 'CUSTOMER' or a 'SUPPLIER'.

You might have, in the PERSON table the unique key of PERSON_ID plus an attribute of PERSON_TYPE ('CUST' or 'SUPP'). If you create the primary key on PERSON_ID,PERSON_TYPE you can reference that in the subtype tables (SUPPLIER/CUSTOMER).

Then you add a unique constraint on the person_id to ensure that any value of person_id must be either a customer or supplier but not both, and check constraints on the subtype tables so that only one type is represented in the table.

create table person
  (person_id     number,
   person_type   varchar2(4),
   name          varchar2(10),
    constraint person_pk primary key (person_id, person_type),
    constraint person_id_uk unique (person_id));

create table supplier
  (supplier_id   number,
   supplier_type varchar2(4),
   blah          varchar2(10),
  constraint supplier_pk primary key (supplier_id, supplier_type),
  constraint supp_pers_fk foreign key  (supplier_id, supplier_type)
    REFERENCES person (person_id, person_type)
  )
/
alter table supplier add constraint supp_type_ck check (supplier_type = 'SUPP');

Its not pretty but types/subtypes are more of an object concept than a relational one.

Answer 4

My solution inspired by Justin:

CREATE OR REPLACE TRIGGER abc
  BEFORE INSERT OR UPDATE ON a
  FOR EACH ROW
  DECLARE
  v_testB NUMBER:= 0;
  v_testC NUMBER:= 0;
  BEGIN
    SELECT
      COUNT(bkey)
    INTO
      v_testB
    FROM
      b
    WHERE
      bkey = :new.aKey;
    SELECT
      COUNT(ckey)
    INTO
      v_testC
    FROM
      c
    WHERE
      ckey = :new.aKey;
    IF ((v_testB + v_testC) <> 1) THEN
      RAISE_APPLICATION_ERROR(-20002,'Foreign key to B or C missing.');
    END IF;
  END;
/
SHOW ERRORS TRIGGER abc

Answer 5

Create a materialized view that unions tables B & C, and point your FK constraint to the view