...javascript注册界面如何判断用户名和邮箱是否已存在,存在则报错...答:sql="INSERT INTO `user`(`id`, `username`, `password`, `email`, `idgroup`, `lastdate`) VALUES ('','$_POST[username]','$_POST[password]','$_POST[email]','$_POST[idgroup]',now())"; $sql1="select * from user where username='$_POST[username]'";...