счета - sht
ID|sheet|date
1|счет1|01.01.2016

товары в счетах items
ID|sheet|item|price
1|счет1|ручка  |100
2|счет1|блокнот|200

оплата по счетам payments
ID|sheet|pay
1|счет1|50
2|счет1|50

счет1 | 300 | 100

SELECT sheet,
SUM(price) AS price_sum,
FROM sht 
LEFT JOIN items USING (sheet)

счет1 | 300

SELECT sheet,
SUM(price) AS price_sum,
SUM(pay) AS pay_sum  
FROM sht 
LEFT JOIN items USING (sheet)
LEFT JOIN payments USING (sheet)

счет1 | 600 | 200

SELECT sheet FROM sht
LEFT JOIN (SELECT SUM(price) AS price_sum FROM items) AS s1 ON sht.sheet = items.sheet
LEFT JOIN (SELECT SUM(pay) AS pay_sum FROM payments)  AS s2 ON sht.sheet = payments.sheet

SELECT sheet,
(SELECT SUM(price) AS price_sum FROM items WHERE sht.sheet = items.sheet) AS s1,
(SELECT SUM(pay) AS pay_sum FROM payments WHERE sht.sheet = payments.sheet)  AS s2
FROM sht;