public function addTableColumn($join, $column, $alias=null)
    {
        $tcol = $this->getTableColumn($join, $column);
        if ($alias) $tcol .= " AS ".$alias;
        if (!$this->_tableColumns)
            $this->_tableColumns = $tcol;
        else
            $this->_tableColumns .= ", ".$tcol;
    }