MYSQL 左联接不包括现有行

Question

我目前已经创建了以下极其简化的 mysql table 来模仿我当前的 tables。

晋级 table:

CREATE TABLE promotion (
  promotion Varchar NOT NULL,
  PRIMARY KEY (promotion)
);

sale_id
10%
20%
30%

和销售 table:

CREATE TABLE sale (
  sale_id int NOT NULL AUTO_INCREMENT,
  promotion Varchar,
  date Date NOT NULL,
  amount int,
  PRIMARY KEY (sale_id)
);

sale_id	promotion	date	amount
1	null	2022-1-1	100
2	10%	2022-1-1	100
3	10%	2022-1-1	100
4	20%	2022-1-1	100
5	null	2022-1-2	100
6	10%	2022-1-2	100
7	30%	2022-1-2	100
8	30%	2022-1-2	100

其中 promotion_id 是升级 table 的外键。所以关系是每次销售可以没有促销（null）或促销（促销主键的int id值作为外键）。

我正在尝试按月显示数据，然后按促销显示数据，其中每个月都有一堆按促销分组的销售金额总计。

到目前为止我有这个查询：

select
    Month(date),
    ifnull(promotion,'No Promo'),
    sum(amount)
from sale
    left join promotion p on p.promotion = sale.promotion
group by Month(date), sale.promotion

这给了我以下结果：

month	promotion	amount
1	null	100
1	10%	200
1	20%	100
2	null	100
2	10%	100
2	30%	200

但我想要的是：

month	promotion	amount
1	null	100
1	10%	200
1	20%	100
1	30%	0
2	null	100
2	10%	100
2	20%	0
2	30%	200

右加入不是意味着将促销列表中的每个促销都包含在促销 table 中吗？不管那个月是否有促销活动？谁能给我一些正确方向的提示，说明为什么我的右连接不起作用？

Answer 1

您需要在联接中反转表的顺序。（右连接也可以，但并非所有版本都支持）

select
    Month(date),
    ifnull(promotion,'No Promo'),
    sum(amount)
from promotion p
    left join sale on p.promotion = sale.promotion
group by Month(date), ifnull(promotion,'No Promo');

Answer 2

我稍微修改了你的查询和你的 table 定义，它在 dbFiddle 中工作正常。

CREATE TABLE promotion (
  promotion Varchar(4) NOT NULL
  PRIMARY KEY
);

✓

INSERT INTO promotion VALUES ('10%'),('20%'),('30%');

✓

CREATE TABLE sale (
  sale_id int NOT NULL PRIMARY KEY  AUTO_INCREMENT,
  promotion Varchar(4),
  date_ Date NOT NULL,
  amount int
);

✓

insert into sale (promotion, date_, amount) values
(null,'2022-1-1',100),
('10%','2022-1-1',100),
('20%','2022-1-1',100),
('30%','2022-1-1',100),
(null,'2022-1-2',100),
('10%','2022-1-2',110),
('20%','2022-1-2',120),
('30%','2022-1-2',130),
(null,'2022-2-1',75),
('10%','2022-2-1',75),
('20%','2022-2-1',75),
('30%','2022-2-1',75),
(null,'2022-2-2',75),
('10%','2022-2-2',75),
('20%','2022-2-2',75),
('30%','2022-2-2',75);

✓

select
    Month(date_) month_,
    ifnull(p.promotion,'No Promo') Promotion,
    sum(amount) Total
from sale
    left join promotion p on p.promotion = sale.promotion
group by Month(date_), ifnull(p.promotion,'No Promo');

month_ | Promotion | Total
-----: | :-------- | ----:
     1 | No Promo  |   200
     1 | 10%       |   210
     1 | 20%       |   220
     1 | 30%       |   230
     2 | No Promo  |   150
     2 | 10%       |   150
     2 | 20%       |   150
     2 | 30%       |   150

db<>fiddle here

Answer 3

我们需要将升级中的约束从主键更改为唯一以允许空值。然后，我们可以使用交叉连接创建所有行，然后左连接到 sale 以检索已售商品的价值。

CREATE TABLE promotion (
  promotion Varchar(4) 
  UNIQUE
);

✓

INSERT INTO promotion VALUES (null),('10%'),('20%'),('30%');

✓

CREATE TABLE sale (
  sale_id int NOT NULL PRIMARY KEY  AUTO_INCREMENT,
  promotion Varchar(4),
  date_ Date NOT NULL,
  amount int
);

✓

insert into sale (promotion, date_, amount) values
(null,'2022-1-1',100),
('10%','2022-1-1',100),
('20%','2022-1-1',100),
('30%','2022-1-1',100),
(null,'2022-1-2',100),
('10%','2022-1-2',110),
('20%','2022-1-2',120),
('30%','2022-1-2',130),
(null,'2022-2-1',75),
('10%','2022-2-1',75),
('30%','2022-2-1',75),
(null,'2022-2-2',75),
('10%','2022-2-2',75),
('30%','2022-2-2',75);

✓

select
    Month(date_) month_,
    ifnull(p.promotion,'No Promo') Promotion,
    sum(amount) Total
from sale
    left join promotion p on p.promotion = sale.promotion
group by Month(date_), ifnull(p.promotion,'No Promo');

month_ | Promotion | Total
-----: | :-------- | ----:
     1 | No Promo  |   200
     1 | 10%       |   210
     1 | 20%       |   220
     1 | 30%       |   230
     2 | No Promo  |   150
     2 | 10%       |   150
     2 | 30%       |   150

select 
  m,
  ifnull(p.promotion,'No Promo') Promotion,
  sum(amount) Total
from (select distinct month(date_) m from sale) m
cross join
promotion p
left join sale s on m.m = month(date_) and p.promotion = s.promotion
group by m, ifnull(p.promotion,'No Promo')
order by m, ifnull(p.promotion,'No Promo');;

 m | Promotion | Total
-: | :-------- | ----:
 1 | 10%       |   210
 1 | 20%       |   220
 1 | 30%       |   230
 1 | No Promo  |  null
 2 | 10%       |   150
 2 | 20%       |  null
 2 | 30%       |   150
 2 | No Promo  |  null

db<>fiddle here

MYSQL 左联接不包括现有行

MYSQL left join include not existing rows

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