表是变异的,触发器/功能可能看不到它(停止平均等级从低于2.5下降)

Question

这是问题所在:

创建一个触发器,以防止对任何特定类中低于2.5的整体平均等级的接受关系进行任何更改.注意:此触发器不是为了解决任何给定学生的平均GPA,而是应该针对特定班级中指定的所有成绩的平均成绩.

这是架构:

Student-schema =(studentnum, name, standing, gpa, major)
Class-schema = (schedulenum, semester, department, classnum, days, time, place, enrollment)
Instructor-schema = (name, department, office)
Teaches-schema = (name, schedulenum, semester)
Taking-schema = (studentnum, schedulenum, semester, grade)

我在这些触发器上度过了一段可怕的时光,但这是我努力做到这一点:

CREATE OR REPLACE TRIGGER stopChange
    AFTER UPDATE OR INSERT OR DELETE ON taking
    REFERENCING OLD AS old
    NEW AS new
    FOR EACH ROW
DECLARE

grd_avg taking.grade%TYPE;

BEGIN
    SELECT AVG(grade)
    INTO grd_avg
    FROM taking
    WHERE studentnum = :new.studentnum
    AND schedulenum = :new.schedulenum
    AND semester = :new.semester;

    IF grd_avg < 2.5 THEN
        UPDATE taking
        SET grade = :old.grade
        WHERE studentnum = :old.studentnum
        AND schedulenum = :old.schedulenum
        AND semester = :old.semester;
    END IF;

END;   
/

我显然做错了,因为当我去更新或删除元组时,我得到错误:

ERROR at line 1:
ORA-04091: table TAKING is mutating, trigger/function may not see it
ORA-06512: at "STOPCHANGE", line 6
ORA-04088: error during execution of trigger 'STOPCHANGE'

有什么建议？我正在使用Oracle.

Answer 1

在里面使用这个语句DECLARE,它会起作用.

pragma autonomous_transaction;

Answer 2

首先,您需要阅读有关触发器,变异表错误和复合触发器的信息:http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/triggers.htm#LNPLS2005

您的触发器是更新或插入或删除后.表示如果在此表上运行UPDATE OR INSERT OR DELETE语句,则触发器将触发.但是你试图在你的触发器内再次更新同一个表,这是compl.错误.这就是您收到错误的原因.您无法修改触发器触发的同一个表.触发器的目的是在您的情况下更新,插入或删除表时自动触发.你需要的是一些程序,而不是触发器.

Answer 3

我认为您可以通过将其重写为前触发器而不是后触发器来解决此问题。但是，这对于插入和删除可能有点复杂。这个想法是：

CREATE OR REPLACE TRIGGER stopChange
    BEFORE UPDATE OR INSERT OR DELETE ON taking
    REFERENCING OLD AS old
    NEW AS new
    FOR EACH ROW
DECLARE

grd_avg taking.grade%TYPE;

BEGIN
    SELECT (SUM(grade) - oldgrade + new.grade) / count(*)
    INTO grd_avg
    FROM taking
    WHERE studentnum = :new.studentnum
    AND schedulenum = :new.schedulenum
    AND semester = :new.semester;

    IF grd_avg < 2.5 THEN
        new.grade = old.grade
    END IF;
END;