任何人都可以告诉我为什么我的SQL查询不起作用(见详情)？

我会稍微重新排列查询....

SELECT
    duplicates.NumOccurrences,
    duplicates.userID,
    users.firstname,
    users.lastname
FROM (
    SELECT
        userID,
        COUNT(userID) AS NumOccurrences
    FROM userDepartments
    GROUP BY userID
    HAVING COUNT(userID) > 1
) duplicates
INNER JOIN users ON duplicates.userID = users.userID