php,mysql如何检测用户是否已经在数据库中？

php,mysql如何检测用户是否已经在数据库中？

$sql=mysql_query(“SELECT FROM users (username, password, email) WHERE username=$fusername”);
if(mysql_num_rows($sql)>=1)
{
echo”name already exists”;
}
else
{
//insert query goes here
}