private function getUpdateQuery()
    {
        if (isset($this->parts['from'][0]['alias'])) {
            $fromClause = $this->parts['from'][0]['alias'].' FROM '.$this->getFromClause();
        } else {
            $fromClause = $this->parts['from'][0]['table'];
        }

        $setClause = array();

        foreach ($this->parts['set'] as $idenfier => $value) {
            $setClause[] = $this->getExpressionBuilder()->equal($idenfier, $value);
        }

        return 'UPDATE '.$fromClause.
               ' SET '.implode(', ', $setClause).
               (($this->parts['where'] !== null) ? ' WHERE '.$this->parts['where'] : null);
    }