Problems With WHERE Clause

Hi there everybody... I'm using Datatables to get data from server trough AJAX request and I'm having some serious issues with WHERE clause and hope you can help me:
a) The Ajax request to get the data, is sent with a "GET" variable, this is being used to Filter the JSON response.
b) The server-side php have the WHERE section configured to filter using the GET variable, and the response is filtered
The problem appears when i use the Search filter, and due to it's config:

$sql_where = "WHERE ID_X=".$_GET['X'];
if ($_GET['sSearch'] != "")
{
    $sql_where = "WHERE ";
    foreach ($columns as $column)
    {
        $sql_where .= $column . " LIKE '%" . mysql_real_escape_string( $_GET['sSearch'] ) . "%' OR ";
    }
    $sql_where = substr($sql_where, 0, -3);
}

It filters from the whole SQL's main query, how can i use the Quick Search to filter from the "already filtered" data?

Thanks a lot!

PS = I've tried to put something like

$sql_where = "WHERE ID_X=".$_GET['X'];
if ($_GET['sSearch'] != "")
{
    $sql_where = "WHERE ID_X=".$_GET['X']." AND";
. . . 

but nothing... Hope this is not a repost, I've search but nothing like this, I know is easy but I just can't see it! Thanks!