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。