In this post, I am sharing a solution on how to allow only single NULL record in the UNIQUE Constraint Column of PostgreSQL.

PostgreSQL: Composite UNIQUE Constraint does not consider NULL Values

Once you define the UNIQUE constraint, you can insert N number of NULL values for that column which is the principal rule of UNIQUE Constraint.

What if I need an only single NULL record for UNIQUE Column, we should check the below demonstration.

Create a sample table with UNIQUE Constraint:

CREATE TABLE tbl_testunique (ID INTEGER UNIQUE);

Insert sample multiple NULLs:
You can insert all NULLs.

INSERT INTO tbl_testunique 
VALUES (1),(NULL),(NULL),(NULL);

Now, Truncate the table:

INSERT INTO tbl_testunique 
VALUES (1),(NULL),(NULL),(NULL);

Create a UNIQUE INDEX with IS NULL:

CREATE UNIQUE INDEX idx_id ON tbl_testunique ((ID IS NULL)) 
WHERE ID IS NULL;

Now, Try to execute same INSERT statement:

INSERT INTO tbl_testunique 
VALUES (1),(NULL),(NULL),(NULL);

Will get ERROR in the result:

********** Error **********
ERROR: duplicate key value violates unique constraint "idx_id"
SQL state: 23505
Detail: Key ((id IS NULL))=(t) already exists.