mysql_query（）非常奇怪地失败了

Question

This is a really simple thing, but it's not working for some reason. Heres my code.

I am making function (its part of a class) which checks if a username or email exists:

public function exists ($what, $who)
{
    $sql = "SELECT * FROM users WHERE $what = $who";
    $query = mysql_query($sql);

    if (mysql_num_rows($query) != 0)
    {
        return true;
    }
    else
    {
        return false;
    }
}

The function returns nothing. In fact if I run that query through regular PHP it returns nothing also. I don't understand why.

This following piece of code returns news entries perfectly:

function fetch($id = '') 
{
    if (empty($id))
    {
        $query = 'SELECT * FROM news ORDER BY id desc';
    }
    elseif (is_numeric($id))
    {
        $query = "SELECT * FROM news WHERE id = $id";
    }
    else
    {
        $route->to(SITE_URL);
    }
    $result = mysql_query($query);

    if (mysql_num_rows($result) > 0)
    {
        return $result;
    }

}

I am confused.

Answer 1

The problem is that you are missing quotes in your query:

 $sql = "SELECT * FROM users WHERE $what = $who";
 //SELECT * FROM users WHERE username = Mario  is not a valid query

should be:

 $sql = "SELECT * FROM users WHERE $what = '$who'";

the other queries are working because you are checking against an id, in this case against a string (and in this case you need quotes)