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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def build_select_fields(self):
 """ Generates the sql for the SELECT portion of the query :return: the SELECT portion of the query :rtype: str """

        field_sql = []

        # get the field sql for each table
        for table in self.tables:
            field_sql += table.get_field_sql()

        # get the field sql for each join table
        for join_item in self.joins:
            field_sql += join_item.right_table.get_field_sql()

        # combine all field sql separated by a comma
        sql = 'SELECT {0}{1} '.format(self.get_distinct_sql(), ', '.join(field_sql))
        return sql