我就直接po截图和代码

也可看我这篇文章

#
alter table testMyIndex add (hometown VARCHAR(45) KEY,money FLOAT UNIQUE); #正确

#报错了，Multiple primary key defined 提示已经有primary key(主键)了
#alter table testMyIndex add (remarks VARCHAR(50) KEY,bankNumber VARCHAR(16) UNIQUE);

报错信息[Err] 1068 - Multiple primary key defined

我记得key关键字可以用来声明普通索引,怎么此时key关键字又有primary key的作用了?

我把创建表的语句拷贝出来

#查看创建表的语句
SHOW CREATE TABLE testMyIndex;

#我把创建表的语句拷贝出来
CREATE TABLE `testmyindex` (
  `xing` varchar(30) DEFAULT NULL,
  `ming` varchar(20) DEFAULT NULL,
  `email` varchar(30) DEFAULT NULL,
  `workCity` varchar(30) DEFAULT NULL,
  `schoolName` varchar(40) DEFAULT NULL,
  `hometown` varchar(45) NOT NULL,
  `money` float DEFAULT NULL,
  PRIMARY KEY (`hometown`),
  UNIQUE KEY `index_email` (`email`),
  UNIQUE KEY `index_workCity2` (`workCity`),
  UNIQUE KEY `index_schoolName_workCity2` (`schoolName`,`workCity`),
  UNIQUE KEY `index_schoolName2` (`schoolName`),
  UNIQUE KEY `money` (`money`),
  KEY `xm` (`xing`,`ming`),
  KEY `index_workCity` (`workCity`),
  KEY `index_schoolName_workCity` (`schoolName`,`workCity`),
  KEY `index_schoolName` (`schoolName`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312