mysql学习之触发器

　　在借阅表和读者表当中存在着这样的关系，如果在借阅表当中添加一条数据，读者表当中对应的累计借书字段就自增1，如果在借阅表当中删除一条数据，读者表当中对应的累计借书字段就自减1，实现本功能的方法如下。

1、创建读者表

 CREATE TABLE `t_borrowbook` (
   `borrowBookId` int(11) NOT NULL AUTO_INCREMENT,
   `bookId` int(11) DEFAULT NULL,
   `id` int(11) DEFAULT NULL,
   `audienceId` int(11) DEFAULT NULL,
   `startTime` datetime DEFAULT NULL,
   `endTime` datetime DEFAULT NULL,
   `borrowBookDesc` text,
   PRIMARY KEY (`borrowBookId`)
 )

2、创建借阅表

 CREATE TABLE `t_audience` (
   `audienceId` int(10) NOT NULL AUTO_INCREMENT,
   `audienceName` varchar(20) NOT NULL,
   `audienceTypeId` int(10) NOT NULL,
   `audienceNumber` varchar(18) NOT NULL,
   `sex` varchar(20) DEFAULT NULL,
   `cellphone` varchar(11) DEFAULT NULL,
   `borrowBookNum` int(10) DEFAULT NULL,
   `audienceDesc` text,
   PRIMARY KEY (`audienceId`)
 )

3、创建插入自增触发器

 DELIMITER $$

 CREATE
     /*[DEFINER = { user | CURRENT_USER }]*/
     TRIGGER `db_book`.`Tr_borrowbook_Insert` AFTER INSERT
     ON `db_book`.`t_borrowbook`
     FOR EACH ROW BEGIN
     UPDATE t_audience SET borrowBookNum = borrowBookNum+1 WHERE audienceId=new.audienceId;
     END$$

 DELIMITER ;

4、创建删除自减触发器

 DELIMITER $$

 CREATE
     /*[DEFINER = { user | CURRENT_USER }]*/
     TRIGGER `db_book`.`Tr_borrowbook_Delete` AFTER DELETE
     ON `db_book`.`t_borrowbook`
     FOR EACH ROW BEGIN
     UPDATE t_audience SET borrowBookNum = borrowBookNum-1 WHERE audienceId=old.audienceId;
     END$$

 DELIMITER ;

　　每天进步一点点，一切都是最好的安排。