您可以使用以下命令(语法更紧凑):
SELECT *
FROM CompletedTasks
WHERE (userID, taskID) NOT IN
( SELECT userID, taskID
FROM PlannedTasks
) ;
或NOT EXISTS
版本(尽管更复杂,但使用适当的索引应该会更有效):
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
) ;
当然还有LEFT JOIN / IS NULL
@jmacinnes的答案中的版本。