public function update($table, $columns) 
    {
        $this->sql = 'UPDATE ' . $this->quoteTableName($table) . ' SET ';
        foreach($columns as $k => $v) {
            $this->sql .= $this->quoteColumnName($k) .' = '. $this->quoteValue($v) . ', ';
        }
        
        $this->sql = trim($this->sql, ', ');
        
        return $this;
    }