PostgreSQL Unique index compound key

Question

I need to create a restriction on a table based on four columns, and two of them are mutually exclusive, I mean, if one is NULL the other is not.

The ideal script would be this:

ALTER TABLE ONLY t_users_prizes
ADD CONSTRAINT t_user_prize_test_circuit_key
UNIQUE (id_event||':'||id_circuit, id_prize, id_user);

But Postgres doesn't allow this kind of concatenation.

How can I implement this restriction?

Answer 1

@Catcall's CHECK constraint enforces that exactly one of (a, b) must be NULL.

But NULL values do not violate a UNIQUE constraint - two NULL are never considered equal! Therefore, the simple UNIQUE constraint doesn't work.

You can get it done with two partial UNIQUE indices:

CREATE TEMP TABLE foo (
   a int
  ,b int
  ,c int NOT NULL
  ,d int NOT NULL
  ,CHECK ((a IS NOT NULL AND b IS NULL) OR (b IS NOT NULL AND a IS NULL))
);

CREATE UNIQUE INDEX foo_acd_idx ON foo(a,c,d)
WHERE b is NULL;

CREATE UNIQUE INDEX foo_bcd_idx ON foo(b,c,d)
WHERE a is NULL;

INSERT INTO foo VALUES (NULL,2,3,4);
INSERT INTO foo VALUES (NULL,2,3,4);  -- error! 

I declared c and d as NOT NULL, to prevent further complications.

More details in this closely related answer.

If you also want to disallow (1, NULL, 3, 4) and (NULL, 1, 3, 4) (see my comment), you could use one index with COALESCE instead:

CREATE UNIQUE INDEX foo_xcd_idx ON foo(COALESCE(a,b),c,d);