How to use multiple queries based on users selection

I have a highcharts chart im using to display number of downloads on apps. The chart is being populated by mysql query.

I have 2 bootstrap dropdown-menu's for that chart. one is date range, and the other is app. The app is populated by a sql query.

Each one of these dropdown's work fine independently. i want them to work together. So if the user chooses a app then a date it will keep track of both.

Here is how im handling the $GET's now

if (isset($_GET['id'])) {
    $app = $_GET['id'];
    $array2 = explode('/', $app);
    $activity = $array2[0];
    $app_name = $array2[1];
    $SQL= " SELECT blah blah blah WHERE activity = ' $activity' ";
} else if (isset($_GET['date'])) {
      $daterange = $_GET['date'];
              if ($daterange = "thisyear"){
               $SQL =
               }
              if ($daterange = "lastyear"){
               $SQL =
               }
} else {
    $SQL = 
}

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I can provide more info if needed.