An important behavior in PostgreSQL to know about is the duplicate null values do not violate unique constraints. Oracle SQL> create table test (id number (2,0), country varchar(20) not null, state varchar(20) ); Table created. SQL> alter table test add constraint pk_test_id primary key (id); Table altered. SQL> alter table test add constraint uniq_test_cs unique … Continue reading PostgreSQL – Unique constraint and null value