public function build() {

		if ( empty( $this->parts ) ) {
			return '';
		}

		$query = '';

		foreach ( $this->parts as $part) {

			if ($part instanceof Builder) {
				$query .= "($part)";
			} elseif ( $sql = (string) $part ) {
				$query .= "$part ";
			}
		}

		return $query;
	}