Construct a SQL query based on the stored blocks.
        :return: a SQL query

def to_sql(self):
        """
        Construct a SQL query based on the stored blocks.
        :return: a SQL query
        """
        return self._sql_query_skeleton.format(
            prefix=self.prefix, select=self.select_block,
            relation=self.from_block, conditions=self.where_block)