这篇文章主要介绍了MySQL触发器概念、原理与用法,结合实例形势详细分析了mysql触发器相关概念、原理、创建、用法及操作注意事项,需要的朋友可以参考下
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navicatcmd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; ;; < pre>
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; ;; < pre>
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 ;
总结
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