PostgreSQL multi-column unique constraint and NULL values

Question

I have a table like the following:

create table my_table (
id   int8 not null,
id_A int8 not null,
id_B int8 not null,
id_C int8 null,
constraint pk_my_table primary key (id),
constraint u_constrainte unique (id_A, id_B, id_C)
);

And I want (id_A, id_B, id_C) to be distinct in any situation. So the following two inserts must result in an error:

INSERT INTO my_table VALUES (1, 1, 2, NULL);
INSERT INTO my_table VALUES (2, 1, 2, NULL);

But it doesn't behave as expected because according to the documentation, two NULL values are not compared to each other, so both inserts pass without error.

How can I guarantee my unique constraint even if id_C can be NULL in this case? Actually, the real question is: can I guarantee this kind of uniqueness in "pure sql" or do I have to implement it on a higher level (java in my case)?

Answer 1

You can do that in pure SQL. Create a partial unique index in addition to the one you have:

CREATE UNIQUE INDEX ab_c_null_idx ON my_table (id_A, id_B) WHERE id_C IS NULL;

This way you can have (1, 2, 1) and (1, 2, 2) and (1, 2, NULL) for (a, b, c) in your table, but none of these a second time.

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Additional notes

• No use for mixed case identifiers without double quotes in PostgreSQL.

• You might consider using a serial column as primary key:

  CREATE TABLE my_table (
    my_table_id bigserial PRIMARY KEY
   ,id_a int8 NOT NULL
   ,id_b int8 NOT NULL
   ,id_c int8
   ,CONSTRAINT u_constraint UNIQUE (id_a, id_b, id_c)
  );
  

If you do not expect more than 2 billion rows (> 2147483647) over the course of time, you might want to use a plain int4 serial column as primary key. Similar considerations for the rest of the columns. Plain integers (int4) are smaller and a bit faster.

Answer 2

A Null can mean that value is not known for that row at the moment but will be added, when known, in the future (example FinishDate for a running Project) or that no value can be applied for that row (example EscapeVelocity for a black hole Star).

In my opinion, it's usually better to normalize the tables by eliminating all Nulls.

In your case, you want to allow NULLs in your column, yet you want only one NULL to be allowed. Why? What kind of relationship is this between the two tables?

Perhaps you can simply change the column to NOT NULL and store, instead of NULL, a special value (like -1) that is known never to appear. This will solve the uniqueness constraint problem.

Answer 3

I had the same problem and I found another way to have unique NULL into the table.

CREATE UNIQUE INDEX index_name ON table_name( COALESCE( foreign_key_field, -1) )

In my case, the field foreign_key_field is a positive integer and will never be -1.

So, to answer Manual Leduc, another solution could be

CREATE UNIQUE INDEX  u_constrainte UNIQUE(COALESCE(id_a, -1), COALESCE(id_b,-1),COALESCE(id_c, -1) )

I assume that ids won't be -1.

What is the advantage on creating a partial index ?
In case where you don't have the NOT NULL clause, id_a, id_b and id_c can be NULL together only once.
With a partial index, the 3 fields could be NULL more than once.