unique constraints

["Campbell, Lance" <lance@xxxxxxxxxxxx>]

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