Question

SELECT DISTINCT L.LecturerName
FROM Lecturer L
WHERE 5 > = 
(   SELECT  E.StudentID
    FROM    Class C, Enrolled E
    WHERE   C.ClassName = E.ClassName
    AND     C.LecturerID = L.LecturerID)

错误消息如下： -

Msg 512，Level 16，State 1，Line 1 子查询返回的值超过1。当子查询遵循=，！=，＆lt;，＆lt; =，＆gt;，＆gt; =或子查询用作表达式时，不允许这样做。

有什么不对吗？

Answer 1

您可以加入主查询Lecturer和Class并应用不同的内容，因此您可以使用少于5名学生（使用子查询）限制您对Enrollment的查询

试试这个：

SELECT DISTINCT L.LecturerName
FROM Lecturer L
JOIN Class C
    ON L.LecturerID = C.LecturerID
WHERE
    (SELECT count(E.ClassName)
     FROM Enrollment E
     WHERE C.ClassName = E.ClassName
     AND C.LecturerID = L.LecturerID) < 5

Answer 2

子查询中需要COUNT(*)。此外，从不在FROM子句中使用逗号。始终使用正确的JOIN语法。

SELECT DISTINCT L.LecturerName
FROM Lecturer L
WHERE 5 >= (SELECT COUNT(*)
            FROM Class C JOIN
                 Enrolled E
                 ON C.ClassName = E.ClassName
            WHERE C.LecturerID = L.LecturerID
           );

我猜测DISTINCT中不需要SELECT。

您没有指定您正在使用的数据库。更典型的情况是，您可以将此信息放在FROM或SELECT条款中，这样您就可以看到学生人数：

SELECT L.LecturerName, COUNT(*) as NumStudents
FROM Lecturer L JOIN
     Class C
     ON C.LecturerID = L.LecturerID JOIN
     Enrolled E
     ON C.ClassName = E.ClassName
GROUP BY L.LecturerName
HAVING COUNT(*) <= 5;

Mini Database- Subquery返回的值超过1

2 个答案: