mysql "Where not in" using two columns

@ypercubeᵀᴹ Thanks for sharing below mention query

SELECT * FROM CompletedTasks WHERE (userID, taskID) NOT IN
      ( SELECT userID, taskID FROM PlannedTasks) ;'

My problem solved.

Is this what you need?

select ct.* from
completedTasks ct
left outer join plannedTasks pt on ct.taskId = pt.TaskId and ct.userId = pt.userId
where pt.taskId is null

However, I agree with the comment - given what we know from the question a status column sounds like a better schema than two tables.

You can use this (more compact syntax):

SELECT *
FROM CompletedTasks
WHERE (userID, taskID) NOT IN
      ( SELECT userID, taskID
        FROM PlannedTasks
      ) ;

or the NOT EXISTS version (which although more complex, should be more efficient with proper indexes):

SELECT c.*
FROM CompletedTasks AS c
WHERE NOT EXISTS 
      ( SELECT 1
        FROM PlannedTasks AS p
        WHERE p.userID = c.userID
          AND p.taskID = c.taskID
      ) ;

and of course the LEFT JOIN / IS NULL version that @jmacinnes has in his answer.