如何检查两个表中某些表中是否存在注册页面中的给定用户名

Question

I want to check if the given username exists in at least one of the two given tables and if it is exists then it will shows them a message that the username already exists. I wrote some code using the UNION statement but it does not show me the message. Can you tell me please what is wrong with my code? Thanks

if (mysql_num_rows(queryMysql("SELECT * FROM doctor
                  WHERE username='$username' UNION SELECT * FROM patient
                  WHERE username='$username'")))
                $error = "That username already exists<br /><br />";
            else
              {
                queryMysql("INSERT INTO doctor (fname,lname,username,email,password,gender,age,specialty,doctorID) VALUES('$fname','$lname','$username','$email', '$password','$DoctorG','$DoctorAge','$specialty','$Doctor_ID')");
                die("<h4>Account created</h4>Please Log in.<br /><br />");
              }

Answer 1

You can try the following:

function exists( $detail, $table, $column, $value ) {
    $stmt = mysql_query("SELECT `$detail` FROM `$table` WHERE `$column` = '$value'");

    if( mysql_num_rows($stmt) >= 1 ) {
        return true;
    } else {
        return false;
    }
}

if( exists('username', 'doctor', 'username', $username) === false && exists('username', 'patient', 'username', $username) === false ) {
    echo 'user doesn\'t exists';
} else {
    echo 'user exists';
}

You should really consider using mysqli_* or PDO instead of mysql_* since it is depracted and will be removed