MySQL 添加外键

MySQL 添加外键
DROP TABLE IF EXISTS `nation`;
CREATE TABLE `nation`(
    `pii_Nation` smallint(2) unsigned NOT NULL default '0',
    `pii_NatinoName` varchar(40) NOT NULL default '',
    PRIMARY KEY  (`pii_Nation`)
)ENGINE=InnoDB DEFAULT CHARSET=utf8;
DROP TABLE IF EXISTS `userinfo`;
CREATE TABLE `userinfo` (
    `ID` char(18) NOT NULL default '',
    `Name` varchar(20) NOT NULL default '',
    `Sex` varchar(2) NOT NULL default '男',
    `pii_Nation` smallint(2) unsigned NOT NULL default '0',
    PRIMARY KEY  (`ID`),
    FOREIGN KEY (`pii_Nation`) references nation(`pii_Nation`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;