protected function _selectCategoryIds($query, $field)
    {
        $oDb = \OxidEsales\Eshop\Core\DatabaseProvider::getDb(\OxidEsales\Eshop\Core\DatabaseProvider::FETCH_MODE_ASSOC);
        $aResult = $oDb->getAll($query);
        $aReturn = [];

        foreach ($aResult as $aValue) {
            $aValue = array_change_key_case($aValue, CASE_LOWER);

            $aReturn[] = $aValue[$field];
        }

        return $aReturn;
    }