I have the following DDL:

CREATE DOMAIN account_nbr_domain VARCHAR(25) NOT NULL;

CREATE TABLE test_table
(
account_nbr account_nbr_domain PRIMARY KEY,
ref_account_nbr account_nbr DEFAULT NULL
);

INSERT INTO test_table (account_nbr, ref_account_nbr) VALUES ('test', NULL);

Firebird throws an exception saying that it cannot insert NULL into ref_account_nbr because of a validation constraint (because the domain is marked NOT NULLABLE).

Is there a way to make the column override the domain nullable constraint?