Because NULL != NULL, therefore you can’t have a unique value for NULL. From: pgsql-admin-owner@xxxxxxxxxxxxxx [mailto:pgsql-admin-owner@xxxxxxxxxxxxxx] On Behalf Of Campbell, Lance PostgreSQL 9.5 I have a table: CREATE TABLE test ( id integer NOT NULL, tag_id integer, CONSTRAINT test_id_tag_id_key UNIQUE (id, tag_id) ) WITH ( OIDS=FALSE ); I noticed I can insert this into the database: Insert into test (id, tag_id) values(1,null); Insert into test (id, tag_id) values(1,null); But this does not work: Insert into test (id, tag_id) values(2,1); Insert into test (id, tag_id) values(2,1); (error occurs) Select * from test; 1, null 1, null 2, 1 I understand why in the second set of inserts I get an error when trying to insert the same set of values. But why does the first example work? I would have thought that it would have only allowed the first insert to work. Thanks, Lance |
