mysql_num_rows返回0，但SQL返回正常值

$getPrd = mysql_query("SELECT * FROM products
LEFT OUTER JOIN productscontent ON productscontent.productsID = products.id
WHERE productscontent.title LIKE '%".$search."%' OR productscontent.alltext LIKE '%".$search."%'
ORDER BY products.sortnr");

$num = mysql_num_rows($getPrd);

And the $getPrd returns values as needed, but the $num returns 0 - why is that?