Postgres Selects with Booleans

gistfile1.txt

v: 9.1.4

if have table users:

id   is_admin
--   --------
1         t
2         f
3          

#true case
select id from users where is_admin 
#=> [1]

#true case
select id from users where not is_admin 
#=> [2]

#undefined
select id from users where is_admin is null
#=> [3]