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深入了解Mysql触发器 一篇文章带你深入了解Mysql触发器

zsr6135 人气:0
想了解一篇文章带你深入了解Mysql触发器的相关内容吗,zsr6135在本文为您仔细讲解深入了解Mysql触发器的相关知识和一些Code实例,欢迎阅读和指正,我们先划重点:Mysql触发器,Mysql深入了解,下面大家一起来学习吧。

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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