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一篇文章带你深入了解Mysql触发器

程序员文章站 2024-01-02 19:01:04
目录1.对sc表进行插入或修改时,如果考试成绩不在0-100范围内时,则撤销插入或修改操作。2.对sc表进行插入时,如果学生的选课总学分超过30,则报错并撤销插入。3.对sc表进行修改时,不允许修改考...

基于school数据库中的student, course, sc表,创建满足下述要求的触发器:

数据库navicat cmd界面运行

1.对sc表进行插入或修改时,如果考试成绩不在0-100范围内时,则撤销插入或修改操作。

delimiter ;
drop trigger if exists `tri_update_grade`;
delimiter ;;
create trigger `tri_update_grade` before update on `sc` for each row begin
if new.grade > 100 then
delete from sc;
elseif new.grade < 0 then
delete from sc;
end if;
end
;;
delimiter ;
drop trigger if exists `tri_insert_grade`;
delimiter ;;
create trigger `tri_insert_grade` before insert on `sc` for each row begin
if new.grade > 100 then
delete from sc;
elseif new.grade < 0 then
delete from sc;
end if;
end
;;

2.对sc表进行插入时,如果学生的选课总学分超过30,则报错并撤销插入。

delimiter ;
drop trigger if exists `tri_insert_credit`;
delimiter ;;
create trigger `tri_insert_credit` before insert on `sc` for each row begin
select sum(ccredit) into @sumcred from student join sc on sc.sno=student.sno join course on course.cno=sc.cno where sc.sno=new.sno;
if (@sumcred>30)
then
delete from sc;
end if;
end
;;

3.对sc表进行修改时,不允许修改考试成绩不及格的学生选课记录。

delimiter ;
drop trigger if exists `tri_grade`;
delimiter ;;
create trigger `tri_grade` before update on `sc` for each row begin
select grade into @gra from sc where sno=new.sno and sno=new.sno;
if (@gra >= 60)
then
set new.grade=20;
else
delete from sc;
end if;
end
;;

4.对student表进行插入时,如果年龄为空,则将该生年龄设置为所有学生年龄的平均值。

delimiter ;
drop trigger if exists `tri_student`;
delimiter ;;
create trigger `tri_student` before insert on `student` for each row begin
select avg(sage) into @avgage from student;
set @old_age = new.sage;
if @old_age is null
then
set new.sage=@avgage;
end if;
end
;;

5.在student表中增加两列,分别用来存放学生的平均成绩和等级,并对sc表进行插入后,自动维护学生的平均成绩和等级(85-100,等级为“优"; 60-85,等级为“良”,60 以下,等级为“差")

delimiter ;
drop trigger if exists `tri_comment`;
delimiter ;;
create trigger `tri_comment` before update on `student` for each row begin
select avg(grade) into @avggra from sc where sno=new.sno;
if @avggra > 85 then
update sc set new.savg=@avggra,new.scomment="you";
else if @avggra > 60 and @avggra <= 85 then
update sc set new.savg=@avggra,new.scomment="you";
else
update sc set new.savg=@avggra,new.scomment='cha';
end if;
end if;
end
;;
delimiter ;

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相关标签: Mysql 触发器

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