如何在 SQL (SQLite) 中编写 SQL 子查询？

Question

这是我正在寻找基于国家/地区的总买入交易价值和总卖出交易价值的示例数据。

这里有两张表，国家和行业 表[公司]：

  +-------------+--------------------+
  |         name|            country |
  +-------------+--------------------+
  |  Alice s.p. |         Wonderland |
  |      Y-zap  |         Wonderland |
  |    Absolute |          Mathlands |
  |  Arcus t.g. |          Mathlands |
  | Lil Mermaid | Underwater Kingdom |
  | None at all |        Nothingland |
  +-------------+--------------------+

表[交易]：

trades:
  +----------+-------------+------------+-------+
  |       id |      seller |      buyer | value |
  +----------+-------------+------------+-------+
  | 20121107 | Lil Mermaid | Alice s.p. |    10 |
  | 20123112 |  Arcus t.g. |      Y-zap |    30 |
  | 20120125 |  Alice s.p. | Arcus t.g. |   100 |
  | 20120216 | Lil Mermaid |   Absolute |    30 |
  | 20120217 | Lil Mermaid |   Absolute |    50 |
  +----------+-------------+------------+-------+

预期输出：

  +--------------------+--------+--------+
  |             country|  buyer |  seller|
  +--------------------+--------+--------+
  |          Mathlands |    180 |     30 |
  |        Nothingland |      0 |      0 |
  | Underwater Kingdom |      0 |     90 |
  |         Wonderland |     40 |    100 |
  +--------------------+--------+--------+

我正在尝试这个：它只给出一个值列，并且它没有显示我想要显示的 0 贸易国家。

select country, sum(value), sum(value) 
from
(select a.buyer as export, a.seller as import, value, b.country as country
from trades as a
join companies as b 
on a.seller=b.name)

group by country 
order by country

请帮忙

Answer 1

将country 加入trades 的不同行，其中仅包含买方或卖方并有条件地聚合：

SELECT c.country,
       SUM(CASE WHEN buyer IS NOT NULL THEN value ELSE 0 END) buyer,
       SUM(CASE WHEN seller IS NOT NULL THEN value ELSE 0 END) seller
FROM country c 
LEFT JOIN (
  SELECT buyer, null seller, value FROM trades
  UNION ALL
  SELECT null, seller, value FROM trades
) t ON c.name IN (t.buyer, t.seller)
GROUP BY c.country

或者，使用 SUM() 窗口函数：

SELECT DISTINCT c.country,
       SUM(CASE WHEN c.name = t.buyer THEN value ELSE 0 END) OVER (PARTITION BY c.country) buyer,
       SUM(CASE WHEN c.name = t.seller THEN value ELSE 0 END) OVER (PARTITION BY c.country) seller
FROM country c LEFT JOIN trades t
ON c.name IN (t.buyer, t.seller)

请参阅demo。

Answer 2

试试 CTE：

WITH sold AS (
    SELECT sum(t.value) AS value, c.country FROM trades AS t INNER JOIN companies AS c ON (t.seller = c.name) GROUP BY c.country
), buyed AS (
    SELECT sum(t.value) AS value, c.country FROM trades AS t INNER JOIN companies AS c ON (t.buyer = c.name) GROUP BY c.country
)
SELECT DISTINCT c.country, COALESCE(b.value, 0) AS buyer, COALESCE(s.value, 0) AS seller
FROM companies AS c 
LEFT JOIN sold AS s ON (c.country = s.country)
LEFT JOIN buyed AS b ON (c.country = b.country)

https://www.db-fiddle.com/f/kgLezmhyiL9BKB2JUsaWYc/0