check if row exists with specific value

I have 2 tables task and taskattributes. There is a linking between 2 tables with taskid. Each taskid has multiple attributes represented by key,value. I would like to find out if specific key exists for the task

For e.g. here if I want to check all the tasks which do not have key ‘A’.

Answer

One simple solution uses aggregation:

SELECT
    t.taskid,
    t.name
FROM task t
INNER JOIN taskattributes ta
    ON t.taskid = ta.taskid
GROUP BY
    t.taskid,
    t.name
HAVING
    COUNT(CASE WHEN "key" = 'A' THEN 1 END) = 0;

If you are using Postgres 9.4 or later, you may use FILTER in the HAVING clause:

HAVING COUNT(*) FILTER (WHERE "key" = 'A') = 0