An article to give you a deep understanding of Mysql triggers

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
;;

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
;;

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
;;

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
;;

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 ;