private static function _extractWhereConditions($conditions) 
    {
        if (!$conditions) {
            return "";
        }

        $cls    = get_called_class();
        $escape = Driver::$escape_char;

        if (is_array($conditions)) {
            // check if is a regular array or an associative one
            if (array_values($conditions) !== $conditions) {
                $conditions = self::_extractWhereAssociativeConditions($conditions, $cls, $escape);
            } else {
                $conditions = self::_extractWhereRegularConditions($conditions, $cls, $escape);
            }
        }
        return $conditions;
    }