如何在postgresql中總結兩列？

Question

我有以下表格：

公司有：

company country
A        us
B        UK
C        france

交易：

seller   buyer   amount
 A         B       10
 B         C       12

我想得到每個國家的買賣總結。 您可以在同一個國家/地區內進行任何交易

country  sum_buyer   sum_seller
us                    10
UK         10         12
france     12

我怎樣才能做到這一點？

Answer 1

一種透視查詢應該執行此任務：

SELECT c.country,
       sum(case when c.company = t.buyer then amount end) as sum_buyer,
       sum(case when c.company = t.seller then amount end) as sum_seller
FROM trades t
JOIN companies c ON c.company IN (t.seller, t.buyer)
GROUP BY c.country
ORDER BY 1 DESC

演示： http ：//www.sqlfiddle.com/#！15/ 7458d2/15

| country | sum_buyer | sum_seller |
|---------|-----------|------------|
|      us |    (null) |         10 |
|      UK |        10 |         12 |
|  france |        12 |     (null) |

Answer 2

為了提高效率，它可能是更好的透視數據，而不是使用in或or在join 。 所以，我建議：

select c.country,
       sum(amount_buyer) as sum_buyer,
       sum(amount_seller) as sum_seller
from trades t cross join lateral
     (values (t.seller, t.amount, 0),
             (t.buyer, 0, t.amount)
     ) v(company, amount_seller, amount_buyer) join
     companies c
     on c.company = v.company
group by c.country;