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

here is my approach :

    Select  * from yourTable where state is null and task_id not in 
    (select task_id from yourTable y where state is not null )