MySQL 获取左连接的总和

Question

我有三张 table 。

1. 地区
2. 美食
3. 横幅

如果我运行此查询

SELECT SUM(fee) FROM BANNERS;

输出将为 10,000

如果我运行此查询

SELECT SUM(fee) FROM CUISINE;

输出将为 12,800

但是如果我运行这个查询

SELECT REGIONS.name,
sum(BANNERS.fee) as banner_revenue,
sum(CUISINE.fee) as cuisine_revenue
FROM REGIONS
LEFT JOIN BANNERS ON REGIONS.id = BANNERS.region_id
LEFT JOIN CUISINE ON REGIONS.id = CUISINE.region_id
GROUP BY REGIONS.name;

输出错误。我想要的输出是

name    | banner_revenue | cuisine_revenue
------------------------------------------
NY      | 10,000         | 4,800
Paris   | NULL           | 8,000
London  | NULL           | NULL

DB fiddle reproduce

为什么会发生这种情况？请引用我的数据库 fiddle 。

Answer 1

如果你运行

SELECT *
       FROM REGIONS
            LEFT JOIN BANNERS
                      ON REGIONS.id = BANNERS.region_id
            LEFT JOIN CUISINE
                      ON REGIONS.id = CUISINE.region_id;

您会注意到，对于每个区域横幅对，所有菜肴都会加入，从而“倍增”菜肴。 IE。他们的费用也会成倍增加。

在派生表中进行分组并将它们连接起来以获得您想要的结果。

SELECT r.name,
       sb.fee,
       sc.fee
       FROM REGIONS r
            LEFT JOIN (SELECT sum(b.fee) fee,
                              b.region_id
                              FROM BANNERS b
                              GROUP BY b.region_id) sb
                      ON sb.region_id = r.id
            LEFT JOIN (SELECT sum(c.fee) fee,
                              c.region_id
                              FROM CUISINE c
                              GROUP BY c.region_id) sc
                      ON sc.region_id = r.id;