Compare values in a table

Hello!

I need to compare two values in a table: cliente and nro_cuota, if both exist I don't want to insert it.

Here is my code

$crud = new grocery_CRUD();

$crud->set_theme('datatables');

$crud->set_table('cobranzas');

$crud->set_subject('Cobranzas');

$crud->set_language('spanish');

$crud->required_fields(
'id',
'cobrador',
'cliente',
'nro_cuota'
);

$crud->columns(
//'id',
'cobrador',
'cliente',
'nro_cuota'
);

$crud->set_relation('cobrador', 'cobradores', 'apellido_nombre');
$crud->set_relation('cliente', 'clientes', 'apellido_nombre');

$output = $crud->render();

$this->load->view('cobranzas/cobranzas_v', $output);

}catch(Exception $e){

show_error($e->getMessage().' --- '.$e->getTraceAsString());
}

}

and I wrote a model, I`m not sure if this is the right way, but I don't know how I must call it

class Compare_values_model extends CI_Model
{

public function compare($id)
{

$this->db->select('"SELECT * FROM `cobranzas` WHERE `cliente` = '$cliente' AND `nro_cuota` = '$nro_cuota'"');

if ($query->num_rows() > 0) {
return true;
} else {
return false;
}

}

}

In pseudo code should be something like thisâ€
if (cliente and nrocuota exists)
{dont insert}
else
{insert}

Hope can help me I couldn't find some examples on internet to learn more, thanks in advance.

lanis