SQL 触发器给出错误。子查询返回超过 1 个值

Question

我有这个数据库，我创建了一个函数来更新学生的 GPA。

CREATE OR ALTER FUNCTION fGPA
    (@Mno varchar(9))
RETURNS DECIMAL(10,4)
BEGIN
    DECLARE @gpa decimal(10,4);

    SET @gpa = (SELECT 
                    (SUM(CASE e.Grade 
                             WHEN 'A' THEN 4.0
                             WHEN 'B' THEN 3.0
                             WHEN 'C' THEN 2.0
                             WHEN 'D' THEN 1.0
                             WHEN 'F' THEN 0.0 
                         END * c.Credit) / SUM(c.Credit)) 
                FROM Enroll AS e
                INNER JOIN Section AS s ON s.Sid = e.Sid
                INNER JOIN Course AS c ON c.Cno = s.Cno
                WHERE @Mno = e.Mno);
    
    RETURN @gpa;
END

我现在正在尝试创建一个触发器，以便每当在 Enroll 中更新成绩时，都会调用 fGPA 函数并同时更新 GPA。这是我所拥有的：

CREATE OR ALTER TRIGGER update_enroll
ON Enroll
AFTER UPDATE
AS
    IF (UPDATE(Grade))
    BEGIN
        DECLARE @updateGPA DECIMAL(10,4);

        SET @updateGPA = (SELECT dbo.fGPA(e.Mno) 
                          FROM Enroll as e
                          INNER JOIN inserted i ON i.Mno = e.Mno);

        UPDATE Student 
        SET student.GPA = @updateGPA;
    END

我得到的错误是：

子查询返回超过 1 个值。当子查询跟随 =、!=、、>= 或子查询用作表达式时，这是不允许的。

当我尝试执行此语句时会发生这种情况

UPDATE ENROLL 
SET Grade = 'B' 
WHERE Sid = '17504';

Answer 1

…
BEGIN
/*
DECLARE @updateGPA DECIMAL(10,4);

SET @updateGPA = (SELECT dbo.fGPA(e.Mno) 
                  FROM Enroll as e
                  INNER JOIN inserted i ON i.Mno = e.Mno);
*/

UPDATE Student 
SET GPA = dbo.fGPA(Mno)
WHERE Mno IN (SELECT i.Mno FROM inserted AS i)

END
…

Answer 2

您正在制作a very common mistake with triggers：inserted 表可能有多个（或零个）行。所以你应该把它作为一个联合更新来做。

请注意，UPDATE() 函数仅告诉您该列是否存在于 UPDATE 语句中，而不是它是否已实际更新。您还应该加入 deleted 表以排除此类行。

这导致了另一个常见问题：使用标量用户定义函数。它们很慢，通常应该避免。相反，请使用 内联 表值函数。

你也在更新整个Student 表，你没有加入任何东西。

CREATE OR ALTER FUNCTION fGPA
    (@Mno varchar(9))
RETURNS TABLE
AS RETURN

    SELECT 
        GPA = SUM(CASE e.Grade 
                 WHEN 'A' THEN 4.0
                 WHEN 'B' THEN 3.0
                 WHEN 'C' THEN 2.0
                 WHEN 'D' THEN 1.0
                 WHEN 'F' THEN 0.0 
             END * c.Credit) / SUM(c.Credit)
    FROM Enroll AS e
    INNER JOIN Section AS s ON s.Sid = e.Sid
    INNER JOIN Course AS c ON c.Cno = s.Cno
    WHERE @Mno = e.Mno;

那你就直接CROSS APPLY吧。

CREATE OR ALTER TRIGGER update_enroll
ON Enroll
AFTER UPDATE
AS
    SET NOCOUNT ON;
    IF (NOT UPDATE(Grade) OR NOT EXISTS (SELECT 1 FROM inserted))  -- bail out for no rows
        RETURN;

    UPDATE s
    SET s.GPA = gpa.GPA
    FROM inserted i
    JOIN Student s ON s.Mno = i.Mno
    CROSS APPLY dbo.fGPA(i.Mno) gpa;

---

我会说我不认为触发器真的是解决这个问题的正确答案。

理想情况下，您不应将相同信息的两份副本保存在两个地方。而是创建一个包含此计算的视图。

CREATE VIEW vGPA
AS

    SELECT
        s.Mno,
        GPA = SUM(CASE e.Grade 
                 WHEN 'A' THEN 4.0
                 WHEN 'B' THEN 3.0
                 WHEN 'C' THEN 2.0
                 WHEN 'D' THEN 1.0
                 WHEN 'F' THEN 0.0 
             END * c.Credit) / SUM(c.Credit)
    FROM dbo.Student AS s
    INNER JOIN dbo.Enroll AS e ON e.Mno = s.Mno
    INNER JOIN dbo.Section AS s ON s.Sid = e.Sid
    INNER JOIN dbo.Course AS c ON c.Cno = s.Cno
    GROUP BY
        s.Mno;

如果您担心性能，可以索引视图以支持这一点。服务器将确保索引始终与基表保持同步。

很遗憾，您不能索引聚合上的计算表达式（即SUM / SUM）。相反，创建一个包含两个 SUMs 的基本视图，然后创建另一个从中选择值的标准视图。

CREATE VIEW vGPA_indexed
WITH SCHEMABINDING  -- needs schema binding
AS

    SELECT
        s.Mno,
        SumGrade = SUM(CASE e.Grade 
                 WHEN 'A' THEN 4.0
                 WHEN 'B' THEN 3.0
                 WHEN 'C' THEN 2.0
                 WHEN 'D' THEN 1.0
                 WHEN 'F' THEN 0.0 
             END * c.Credit),
        SumCredit = SUM(c.Credit),
        TotalRows = COUNT_BIG(*)  -- needs count in grouped view
    FROM dbo.Student AS s
    INNER JOIN dbo.Enroll AS e ON e.Mno = s.Mno
    INNER JOIN dbo.Section AS s ON s.Sid = e.Sid
    INNER JOIN dbo.Course AS c ON c.Cno = s.Cno
    GROUP BY
        s.Mno;

CREATE UNIQUE CLUSTERED INDEX CX_vGPA_indexed on vGPA_indexed (Mno);

CREATE VIEW dbo.vGPA
AS

    SELECT
        s.Mno,
        GPA = SumGrade / SumCredit
    FROM dbo.vGPA_indexed WITH (NOEXPAND);  -- needs NOEXPAND in standard edition, and anyway better for performance