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

As COUNT function ignores NULL in case a specific column is assigned, you can use group by, count and having as below.

SELECT task_id
FROM t1
GROUP BY task_id
HAVING count(STATE) = 0;

DEMO