Autocomplete field with jQuery, Ajax and PHP

Greetings, I have a DAO.php where I declare the functions that make the requests to the BD, this function in particular:

function select_all_unis(){
        $tecla_pulsada = $_POST['service'];
        $sql = "SELECT * FROM dumies WHERE studies LIKE '".$tecla_pulsada."%'";

        $conexion = connect::con();
        $res = mysqli_query($conexion, $sql);
        connect::close($conexion);
        return $res;
    }

And then in a controller (controller.php) with different 'case', specifically the one I use is:

case 'list_studies':
        try{
            $daolawyer = new DAOShop();
            $rdo = $daolawyer->select_all_unis();
        }catch (Exception $e){
            echo json_encode("error");
            exit;
        }
        if(!$rdo){
            echo json_encode("error");
            exit;
        }else{
            while($lawyer = mysql_fetch_array($rdo)){
                echo '<div>
                        <a class="suggest-element" data="'.$lawyer['studies'].'" id="service'.$lawyer['dni'].'">'.utf8_encode($lawyer['studies']).'</a>
                    </div>';                   
            }
            exit;
        }
        break;

What is called in a js:

$(document).ready(function() {    
//Al escribr dentro del input con id="service"
$('#service').keypress(function(){
    //Obtenemos el value del input
    var service = $(this).val();        
    var dataString = 'service='+service;

    //Le pasamos el valor del input al ajax
    $.ajax({
        type: "POST",
        url: "module/shop/controller/controller_shop.php?op=list_studies",
        data: dataString,
        success: function(data) {
            //Escribimos las sugerencias que nos manda la consulta
            $('#suggestions').fadeIn(1000).html(data);
            //Al hacer click en algua de las sugerencias
            $('.suggest-element').on('click', function(){
                //Obtenemos la id unica de la sugerencia pulsada
                var id = $(this).attr('id');
                //Editamos el valor del input con data de la sugerencia pulsada
                $('#service').val($('#'+id).attr('data'));
                //Hacemos desaparecer el resto de sugerencias
                $('#suggestions').fadeOut(1000);
            });              
        }
    });
});              

});

And this is the html code.

<form>
    <input type="text" size="50" id="service" name="service" />
        <div id="suggestions"></div>
</form>

I just need to know how depending on the keys that the BD press returns me some values or others, for example the BD I have a series of users, with the universities where they studied, and I want that by pressing the key c the BD give me back the universities that start with c. I understand that it would be something like $ _POST ['services]; but I do not know where to put it exactly, the code that I have based on is: link But of course my model structure, controller view is different.