How to select grouped rows with only NULL values?

Question

If I have a table like

task_id | state
--------+------------ 
   1    |  NULL
--------+------------
   1    |  RESOLVED
--------+------------
   2    |  NULL
--         


        

Answer 1

Do a GROUP BY, use HAVING to return task_id having only null states.

select task_id
from tablename
group by task_id
having max(state) is null