SELECT
	c.VATIN,
	SUM(
		SELECT value_NET * tax_rate/100
		FROM Invoice
		WHERE c.VATIN = seller_VATIN AND date < s.end_date AND date > s.start_date)
		- SUM(
			SELECT value_NET * tax_rate/100
			FROM Invoice
			WHERE c.VATIN = buyer_VATIN AND date < s.end_date AND date > s.start_date),
	s.start_date
FROM Client c, Settling_period s
GROUP BY c.VATIN, s.start_date