Function on API

7

So I changed to this. $rfechas=mysqli_query($sqlfechas); and I am getting a response from my code, but getting this errors
PHP Warning: mysqli_query() expects at least 2 parameters, 1 given and

PHP Warning:  mysqli_fetch_row() expects parameter 1 to be mysqli_result, null given

My code is this.

$cups=$_POST['cups'];
$sqlfechas="some sql";
$rfechas=mysqli_query($sqlfechas);
$rrfechas=mysqli_fetch_row($rfechas);
$fecha1=$rrfechas[0];
$fecha2=$rrfechas[1];

if (mysqli_num_rows($rfechas)>0){

    $electricidad=calcular_termino_electricidad($cups,$fecha1,$fecha2);
    if (is_array($electricidad)){
        foreach ($electricidad as $periodo) {
            # code...
            foreach ($periodo as $id_periodo => $fechas) {
                # code...
                foreach ($fechas as $fecha => $datos) {
                    # code...
                    $consumoTotal = $consumoTotal + $datos['consumo'];
                }
            }

        }
        echo json_encode(array('error'=>0, 'lectura'=>$consumoTotal,'fecha_ini'=>$fecha1,'fecha_fin'=>$fecha2));
 echo json_encode(array('error'=>0, 'lectura'=>$consumoTotal,'fecha_ini'=>$fecha1,'fecha_fin'=>$fecha2));
    }
    }else{
    echo json_encode(array('error'=>1));
}

and i am getting error 1, question is now (I know nothing about PHP haha) i am getting echo error 1 because my query doesnt find any data, or because i am not given the mysqli_connect params to mysqli_query and mysqli_fetch_rows?