如下，有四张表：游戏类型表，游戏表，点卡和游戏关系表，点卡表
CREATE TABLE `gamesType`(
`tId` INT AUTO_INCREMENT NOT NULL PRIMARY KEY COMMENT\'游戏类型编号\',
`tName` VARCHAR(10) NOT NULL COMMENT\'游戏类型名称\'
)ENGINE=INNODB DEFAULT CHARSET=utf8 COMMENT=\'游戏类型表\';
CREATE TABLE `games`(
`gid` INT AUTO_INCREMENT NOT NULL PRIMARY KEY COMMENT\'游戏编号\',
`gName` VARCHAR(10) NOT NULL COMMENT\'游戏名称\',
`tId` INT NOT NULL COMMENT\'所属游戏类型\'
)ENGINE=INNODB DEFAULT CHARSET=utf8 COMMENT=\'游戏表\';
CREATE TABLE `gameOrCard`(
`id` INT AUTO_INCREMENT NOT NULL PRIMARY KEY COMMENT\'编号\',
`cid` INT NOT NULL COMMENT\'点卡编号\',
`gId` INT NOT NULL COMMENT\'支持支付游戏编号\'
)ENGINE=INNODB DEFAULT CHARSET=utf8 COMMENT=\'点卡和游戏关系表\';
CREATE TABLE `cards`(
`cId` INT AUTO_INCREMENT NOT NULL PRIMARY KEY COMMENT\'点卡编号\',
`cName` VARCHAR(20) NOT NULL COMMENT\'点卡名称\',
`titleImg` VARCHAR(20) NOT NULL COMMENT\'点卡图标\',
`iniPrice` DOUBLE NOT NULL COMMENT\'原价格\',
`price` DOUBLE NOT NULL COMMENT\'售价\',
`cInfo` VARCHAR(200) NOT NULL COMMENT\'点卡介绍\',
`praiseRate` DOUBLE NOT NULL COMMENT\'好评率\',
`upTime` TIMESTAMP NOT NULL COMMENT\'上架时间\',
`isShelves` INT(1) NOT NULL COMMENT\'上架状态0上架1下架\',
`number` INT NOT NULL COMMENT\'数量\'
)ENGINE=INNODB DEFAULT CHARSET=utf8 COMMENT=\'点卡表\';
上述已经建立好了主外键关系。
要查找“游戏类型为网络游戏的游戏信息及其支持该游戏充值得点卡信息，并且该点卡为上架状态”：
SELECT c.*,g.* FROM `cards` AS c JOIN `gameOrCard` AS ga ON c.cid=ga.cid JOIN `games` AS g ON g.`gid`=ga.`gid` WHERE `tid`=1 AND c.isShelves=0 ORDER BY g.gid;