MYSQL select 具有预定义值的数据，如果日期不存在于同一 table

Question

我的结果 sql 语法为：

SELECT AreaName, Date, Total
FROM table
GROUP BY AreaName, Date

此处结果：

AreaName	Date	Total
London	2021-01-01	2
Paris	2021-01-01	3
London	2021-02-01	4
Beijing	2021-02-01	5

但如果日期数据不存在且总计值为 0，我想包括 AreaName。像这里一样：

AreaName	Date	Total
London	2021-01-01	2
Paris	2021-01-01	3
Beijing	2021-01-01	0
London	2021-02-01	4
Paris	2021-02-01	0
Beijing	2021-02-01	5

有什么建议吗？

Answer 1

WITH
areas AS ( SELECT DISTINCT AreaName 
           FROM source_table ),
dates AS ( SELECT DISTINCT `Date`
           FROM source_table )
SELECT AreaName, `Date`, COALESCE(Total, 0) Total
FROM areas
CROSS JOIN dates
NATURAL LEFT JOIN source_table;

Answer 2

我们可以使用 sub-queries 和 DISTINCT 来获取所有日期和所有 AreaNames 的列表，然后交叉连接它们。
警告：这会使值相乘，并可能快速创建一个包含大量零值的巨大结果集。

create table Areas(
AreaName varchar(25),
AreaDate Date,
Total int);
insert into Areas values
('London', '2021-01-01',  2),
('Paris',  '2021-01-01',  3),
('London', '2021-02-01',  4),
('Beijing','2021-02-01',  5);

select
  a.AreaName,
  b.AreaDate,
  coalesce(c.Total,0) "Total"
from
(select distinct AreaName from Areas) a
cross join 
(select distinct AreaDate from Areas) b
left join Areas c
on a.AreaName = c.AreaName
and b.AreaDate = c.AreaDate
order by AreaDate, AreaName

AreaName | AreaDate   | Total
:------- | :--------- | ----:
Beijing  | 2021-01-01 |     0
London   | 2021-01-01 |     2
Paris    | 2021-01-01 |     3
Beijing  | 2021-02-01 |     5
London   | 2021-02-01 |     4
Paris    | 2021-02-01 |     0

db<>fiddle here

MYSQL select 具有预定义值的数据，如果日期不存在于同一 table

MYSQL select data with predefined values if date not exist in same table

mysql

database

join

union