Postgres SQL Exclusive OR (XOR) CHECK CONSTRAINT, is it possible?

Question

Is it possible to make a XOR CHECK CONSTRAINT?

I'm doing it on a test table I just made that is called test and has 3 columns:

• id, bigint
• a, bigint
• b, bigint

I made a check constraint for this:

(a IS NOT NULL AND b = NULL) OR (b IS NOT NULL AND a = NULL)

Which apparently would work in MSSQL

I tested it by doing this:

INSERT INTO public.test(
    id, a, b)
    VALUES (1, 1, 1);

Which should fail, seeing as it doesn't evaluate to TRUE on either side of the OR. However, it's inserting just fine.

When I look at what postgres actually stored as constraint I get this:

(a IS NOT NULL AND b = NULL::bigint OR b IS NOT NULL AND a = NULL::bigint)

I heard AND takes precedent over OR so even this should still work.

Does anyone have a solution for this? Preferably one that is also possible with three or more columns? I understand that those might be more complicated though.

EDIT: Changing

= NULL

to

IS NULL

give me:

ERROR:  cannot cast type boolean to bigint

Answer 1

Right, the a = NULL and b = NULL bit was the issue as @a_horse_with_no_name indicated. You might also consider this derivative, which doesn't require the OR operator:

create table test 
(
  id integer primary key, 
  a integer, 
  b integer, 
  check ((a IS NULL) != (b IS NULL))
);

Of course that works exclusively with only two column XOR comparison. With three or more column XOR comparison in a similar test table you could resort to a similar approach more like this:

create table test 
(
  id integer primary key, 
  a integer, 
  b integer, 
  c integer, 
  check ((a IS NOT NULL)::INTEGER + 
         (b IS NOT NULL)::INTEGER + 
         (c IS NOT NULL)::INTEGER = 1)
);