一、创建触发器的基本语法：
CREATE TRIGGER 触发器名 BEFORE|AFTER 触发事件 ON 表名 FOR EACH ROW 触发器语句
触发事件包括：INSERT、UPDATE、DELETE
二、正确案例演示
如果想在sql文件中直接创建触发器，下面为一个完整的sql文件创建（含触发器的创建）示例：
- use mysql
- drop database if exists myTest;
- create database myTest default charset=gb2312;
- use myTest;
- create table Stud
- (
- sno int(8),
- sname varchar(64),
- ssex varchar(64),
- sage int(4) ,
- sdep varchar(128)
- );
- create table Scsc
- (
- sno int(8),
- cno int(4),
- grade int(4)
- );
- insert into Stud(sno,sname,ssex,sage,sdep) values(2006126001,'lwj','female',19,'com scen');
- insert into Stud(sno,sname,ssex,sage,sdep) values(2006126002,'fj','male',20,'com scen');
- insert into Scsc(sno,cno,grade) values(2006126001,1001,70);
- insert into Scsc(sno,cno,grade) values(2006126002,1002,85);
- -- 注意下面mysql的触发器的用法:
- -- 当其放在sql语句中与数据库/表一起创建的时候
- -- 必须重定义结束符为$$(代替;),否则创建出错。
- -- 且创建触发器的过程要放在quit的前面，这样
- -- 不至于造成常规的sql语句无法执行问题。
- delimiter $$
- drop trigger if exists delTrigger$$
- create trigger delTrigger after delete on Stud for each row
- begin
- declare oldsno char(10);
- set oldsno=old.sno;
- delete from Scsc where sno=oldsno;
- end$$
- drop trigger if exists updateTgr$$
- create trigger updateTgr after update on Stud for each row
- begin
- declare newsno char(10);
- declare oldsno char(10);
- set newsno = NEW.sno;
- set oldsno = OLD.sno;
- if newsno>oldsno then
- update Scsc set sno=newsno where sno=oldsno;
- end if;
- end$$
- quit
图1 创建成功
图2 更新触发测试
三、易错问题
而下面的写法虽然语法上没有问题，但是却无法直接写到sql文件中的！原因不明！
- ------- Error usage
- drop trigger if exists delTrigger
- create trigger delTrigger after delete on Stud for each row
- begin
- declare oldsno char(10);
- set oldsno=old.sno;
- delete from Scsc where sno=oldsno;
- end
- drop trigger if exists updateTgr
- create trigger updateTgr after update on Stud for each row
- begin
- declare newsno char(10);
- declare oldsno char(10);
- set newsno = NEW.sno;
- set oldsno = OLD.sno;
- if newsno>oldsno then
- update Scsc set sno=newsno where sno=oldsno;
- end if;
- end
- ------- Error usage
图3
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