public function build()
    {
        /**
         * Query to be returned.
         *
         * @var string
         */
        $query = '';

        /**
         * Clauses which were built already.
         *
         * It is required to keep track of built clauses because some fields,
         * for example `join` is used by multiple clauses (`JOIN`, `LEFT JOIN`,
         * `LEFT OUTER JOIN`, etc.). The same happens for `VALUE` and `VALUES`.
         *
         * A clause is considered built just after fields' value
         * (`$this->field`) was used in building.
         *
         * @var array
         */
        $built = [];

        /**
         * Statement's clauses.
         *
         * @var array
         */
        $clauses = $this->getClauses();

        foreach ($clauses as $clause) {
            /**
             * The name of the clause.
             *
             * @var string
             */
            $name = $clause[0];

            /**
             * The type of the clause.
             *
             * @see self::$CLAUSES
             *
             * @var int
             */
            $type = $clause[1];

            /**
             * The builder (parser) of this clause.
             *
             * @var Component
             */
            $class = Parser::$KEYWORD_PARSERS[$name]['class'];

            /**
             * The name of the field that is used as source for the builder.
             * Same field is used to store the result of parsing.
             *
             * @var string
             */
            $field = Parser::$KEYWORD_PARSERS[$name]['field'];

            // The field is empty, there is nothing to be built.
            if (empty($this->$field)) {
                continue;
            }

            // Checking if this field was already built.
            if ($type & 1) {
                if (! empty($built[$field])) {
                    continue;
                }
                $built[$field] = true;
            }

            // Checking if the name of the clause should be added.
            if ($type & 2) {
                $query = trim($query) . ' ' . $name;
            }

            // Checking if the result of the builder should be added.
            if ($type & 1) {
                $query = trim($query) . ' ' . $class::build($this->$field);
            }
        }

        return $query;
    }