drupal=> create table foo (bar int, baz text not null);
CREATE TABLE
drupal=> \d foo
      Table "public.foo"
 Column |  Type   | Modifiers 
--------+---------+-----------
 bar    | integer | 
 baz    | text    | not null

drupal=> insert into foo (bar) values (1);
ERROR:  null value in column "baz" violates not-null constraint