sql - PostgreSQL IS NULL and length -

i trying records table specific column null. not getting records. on other hand, there many records length(field) indeed 0.

select count(*) article length(for_interest) =0;  count  -------      9 (1 row)  select count(*) article for_interest null ;  count  -------      0 (1 row) 

something nulls didn't right? more info

select count(*) article for_interest not null ;  count  -------     15 (1 row)  select count(*) article ;  count  -------     15 (1 row) 

postgresql version 9.3.2.

adding sample data, table description etc (new sample table created 2 records this)

test=# \d sample_article             table "public.sample_article"     column    |          type          | modifiers  --------------+------------------------+-----------  id           | integer                |   title        | character varying(150) |   for_interest | character varying(512) |   test=# select * sample_article length(for_interest)=0;  id |                              title                              | for_interest  ----+-----------------------------------------------------------------+--------------   8 | logic behind popular interview questions?  |  (1 row)  test=# select * sample_article for_interest not null;  id |                              title                              | for_interest  ----+-----------------------------------------------------------------+--------------   7 | auto expo 2014: bike gallery                                    | wheels   8 | logic behind popular interview questions?  |  (2 rows)  test=# select * sample_article for_interest null;  id | title | for_interest  ----+-------+-------------- (0 rows) 

character types can hold empty string '', not null value.
length of empty string 0. length of null value null.
functions return null null input.

select length('');   --> 0 select length(null); --> null  select null null; --> true select '' null;   --> false