带有聚合函数的PL / SQL触发器

Posted 2021-05-05

我有一个触发器，它使用avg()函数根据course表中的课程分数来计算学生的gpa。

这里的问题是，当我的触发器执行时，表中的GPA参数为空。

谁能看到这个问题？

drop table courses;
drop table student;
drop table assignments;

create table student (sid integer, sname char(10), saddress char(10), gpa integer);
create table courses (sid integer, cid integer, cgrade integer);
create table assignments ( sid integer, cid integer, aid integer, agrade integer);


create or replace trigger updategpa before insert or update of cgrade on courses for each row
    begin
        if inserting then
            update student set gpa = (select avg(cgrade) from courses where courses.sid = student.sid) where sid = :new.sid;
        elsif inserting then
            update student set gpa = (select avg(cgrade) from courses where courses.sid = student.sid) where sid = :new.sid;
        elsif deleting then
            delete from student where sid = :new.sid;
        end if;
    end;
/
show errors;


insert into student (sid, sname, saddress, gpa) values (1, 'Mike', 'Brighton', 0);
insert into courses (sid, cid, cgrade) values(1, 2550, 0);
insert into assignments values(1, 2550, 1, 70);
insert into assignments values(1, 2550, 2, 80);

select * from courses;
select * from student;
select * from assignments;

update assignments set agrade = agrade + 5;

select * from courses;
select * from student;
select * from assignments;

答案

根据知识，您的触发器似乎是正确的。但是如果你仔细看看你的插入语句。你正在插入insert into courses (sid, cid, cgrade) values(1, 2550, 0);这里cgrade的值是0.所以很明显avg（0）是0并且将gpa更新为0.如果你插入几个值，你肯定会在学生表格中以gpa更新avg（CGRADE）。

尝试更多插入来检查像insert into courses (sid, cid, cgrade) values(1, 2551, 10); insert into courses (sid, cid, cgrade) values(1, 2551, 15); insert into courses (sid, cid, cgrade) values(1, 2551, 20);

你肯定会得到结果：

1 Mike Brighton 8