How to make two isset-functions that work together in PHP?

How to make two if-functions that work together in PHP?

In my code i have different buttons that allow the user to sort the way he wants to sort the rows from the database, they click a button, and the sql-statement changes like this:

        if (isset($_GET['ASC'])){
            $sql = "SELECT * FROM merke JOIN modell ON merke.idmerke=modell.merke_idmerke JOIN modell_has_personer ON modell.idmodell=modell_has_personer.modell_idmodell JOIN personer ON modell_has_personer.personer_idpersoner=personer.idpersoner ORDER BY merkenavn ASC";}
        else if (isset($_GET['aar'])){$sql = "SELECT * FROM merke JOIN modell ON merke.idmerke=modell.merke_idmerke JOIN modell_has_personer ON modell.idmodell=modell_has_personer.modell_idmodell JOIN personer ON modell_has_personer.personer_idpersoner=personer.idpersoner ORDER BY aar ASC";} 
            else {$sql = "SELECT * FROM merke JOIN modell ON merke.idmerke=modell.merke_idmerke JOIN modell_has_personer ON modell.idmodell=modell_has_personer.modell_idmodell JOIN personer ON modell_has_personer.personer_idpersoner=personer.idpersoner ORDER BY merkenavn DESC";}
        $resultat =  $kobling->query($sql);

Here ASC and aar are different buttons that the user can click on, and when he does, the sql-statement changes.

This works well, but then i wanted to implement an option for the user to sort out rows that doesnt start with the letters that the user chooses like this:

if (isset($_GET['go'])){ $ja = $_GET["ja"]; 
    $sql = "SELECT * FROM merke JOIN modell ON merke.idmerke=modell.merke_idmerke JOIN modell_has_personer ON modell.idmodell=modell_has_personer.modell_idmodell JOIN personer ON modell_has_personer.personer_idpersoner=personer.idpersoner WHERE merkenavn LIKE '$ja%'";}

Here $ja is the variable for what letter(s) the rows must start on.

My problem her is this: How to i combine these to so that if the user clicks on a button to sort the rows but also wants to sort out rows by first letter(s) both of them happen?