private function getSqlGroupBy() {
$zoneColumnName = Gpf_Common_DateUtils::getSqlTimeZoneColumn('t.'.Pap_Stats_Table::DATEINSERTED);
switch($this->timeGroupBy) {
case Gpf_Common_DateUtils::DAY: return "TO_DAYS($zoneColumnName)";
case Gpf_Common_DateUtils::WEEK: return "YEAR($zoneColumnName), WEEK($zoneColumnName)";
case Gpf_Common_DateUtils::MONTH: return "YEAR($zoneColumnName), MONTH($zoneColumnName)";
case Gpf_Common_DateUtils::YEAR: return "YEAR($zoneColumnName)";
}
}