$selected_services = array(512, 825, 912, 913, 936);

$query = db_select('users', 'u');
$query->join('users_roles', 'ur', 'ur.uid = u.uid');
$query->join('role', 'r', 'r.rid = ur.rid');
$query->condition('r.name', 'customer');
$query->join('field_data_field_topic_activity', 'service', 'u.uid = service.entity_id');
$query->condition('service.field_topic_activity_tid', $selected_services, 'IN');
$query->groupBy('u.uid');
$query->fields('u', array('uid'))->condition('u.status', 1);

$result = $query->execute()->fetchField();

$query = db_select('users', 'u');
$query->join('users_roles', 'ur', 'ur.uid = u.uid');
$query->join('role', 'r', 'r.rid = ur.rid');
$query->condition('r.name', 'customer');
$query->join('field_data_field_topic_activity', 'service', 'u.uid = service.entity_id');
$query->condition('service.field_topic_activity_tid', $selected_services, 'IN');
$query->groupBy('u.uid');
$query->fields('u', array('uid'))->condition('u.status', 1);

if(!empty($cat)) {
  $query->join('field_data_field_my_product', 'collection', 'u.uid = collection.entity_id');
  $query->join('field_data_field_sub_category', 'product', 'product.entity_id = collection.field_my_product_value');
  $query->condition('product.field_sub_category_tid', $cat, 'IN');
}

if(!empty($city)) {
  $query->join('field_data_field_address_website', 'address_collection', 'u.uid = address_collection.entity_id');
  $query->join('field_data_field_user_address', 'address', 'address.entity_id = address_collection.field_address_website_value');
  $query->condition('address.field_user_address_administrative_area', $city, 'IN');
}
$query->range(($pager - 1) * 20, 20);
$result = $query->execute();