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

Answer 2

Try this -

SELECT task_id
FROM YOUR_TABLE
WHERE state NOT IN (SELECT task_id
                    FROM YOUR_TABLE
                    WHERE task_id IS NOT NULL)

Answer 3

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 )

Answer 4

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