什么是觸發(fā)器？

觸發(fā)器是數(shù)據(jù)庫(kù)的一個(gè)程序，他是用來(lái)監(jiān)聽(tīng)著數(shù)據(jù)表的某個(gè)行為，一旦數(shù)據(jù)表的這個(gè)行為發(fā)生了，馬上執(zhí)行相應(yīng)的sql語(yǔ)句

觸發(fā)器的語(yǔ)法結(jié)構(gòu)：

create trigger 觸發(fā)器的名稱觸發(fā)器事件 on 監(jiān)聽(tīng)的表名 for each row 行為發(fā)生后執(zhí)行的sql語(yǔ)句

觸發(fā)器事件組成：；兩部分組成：

觸發(fā)器事件發(fā)生的時(shí)間-----是在監(jiān)聽(tīng)的表的行為 after before 常用的是after

觸發(fā)器執(zhí)行的內(nèi)容：增刪改

創(chuàng)建order 表的時(shí)候，需要注意，因?yàn)閛rder在mysql中是一個(gè)關(guān)鍵字排序，為了避免錯(cuò)誤的發(fā)生，我們可以添加反引號(hào)，表明這不是一個(gè)關(guān)鍵字

案例研究：

一旦生成訂單，對(duì)應(yīng)的庫(kù)存表要減去相應(yīng)的數(shù)據(jù)

（1）建兩張表 ：一個(gè)商品goods表 一個(gè)訂單order表
mysql> create table goods(goods_id int primary key auto_increment,goods_name var
char(64),shop_price decimal(10,2),goods_number int)engine=mysiam default charset
=utf8;

mysql> create table `order`(goods_id int primary key auto_increment,goods_name v
archar(64),buy_number int)engine=mysiam default charset=utf8;

mysql> insert into goods values(null,'nokiaN85',2000,35),(null,'iphone4S',4500,3
0),(null,'Lnmia',5000,40),(null,'samsung',4200,20);

mysql> select*from goods;
+----------+------------+------------+--------------+
| goods_id | goods_name | shop_price | goods_number |
+----------+------------+------------+--------------+
| 1 | nokiaN85 | 2000.00 | 35 |
| 2 | iphone4S | 4500.00 | 30 |
| 3 | Lnmia | 5000.00 | 40 |
| 4 | samsung | 4200.00 | 20 |
+----------+------------+------------+--------------+

（2）創(chuàng)建觸發(fā)器
mysql> create trigger alter_goods_number after insert on `order` for each row up
date goods set goods_number=goods_number-5 where goods_id=1;

mysql> insert into `order` values(1,'nokiaN85',5);

mysql> select*from goods;
+----------+------------+------------+--------------+
| goods_id | goods_name | shop_price | goods_number |
+----------+------------+------------+--------------+
| 1 | nokiaN85 | 2000.00 | 30 |
| 2 | iphone4S | 4500.00 | 30 |
| 3 | Lnmia | 5000.00 | 40 |
| 4 | samsung | 4200.00 | 20 |
+----------+------------+------------+--------------+


new 的使用 mysql> create trigger alter_goods_number after insert on `order` for each row up
date goods set goods_number=goods_number-new.buy_number where goods_id=new.goods
_id;

mysql> insert into `order` values(4,'samsung',5);

old 的使用

mysql> create trigger back_goods_number after delete on `order` for each row upd
ate goods set goods_number=goods_number+old.buy_number where goods_id=old.goods_
id;

mysql> delete from `order` where goods_id=1;

更新 (update將之前下的訂單撤銷，再重新下訂單)

mysql> create trigger update_goods_number after update on `order` for each row u
pdate goods set goods_number=goods_number+old.buy_number-new.buy_number where go
ods_id=new.goods_id;

mysql> update `order` set buy_number = 10 ;

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