Mysql group by weekday，补缺的工作日

Question

我的 MySQL 查询有问题，它按工作日对表中的数据进行分组。

我需要它来填写数据中缺失的工作日，例如下面 SQL 示例中的星期日(第 7 个工作日)。

SQL Fiddle

MySQL 5.6 架构设置:

create table test (
`id` INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
`date` DATETIME
);

INSERT INTO test (`date`) VALUES 
('2016-05-16 00:00:00'),
('2016-05-17 00:00:00'),
('2016-05-18 00:00:00'),
('2016-05-20 00:00:00'),
('2016-05-21 00:00:00'),
('2016-05-22 00:00:00'),
('2016-05-16 00:00:00'),
('2016-05-17 00:00:00'),
('2016-05-18 00:00:00'),
('2016-05-20 00:00:00');

查询 1:

SELECT WEEKDAY(date) AS weekday,
    COUNT(id) AS posts
FROM test
GROUP BY WEEKDAY(date)

Results :

| weekday | posts |
|---------|-------|
|       0 |     2 |
|       1 |     2 |
|       2 |     2 |
|       4 |     2 |
|       5 |     1 |
|       6 |     1 |

我希望它也返回这一行。

|       3 |     0 |

我的完整查询非常复杂，所以我希望您能找到一个快速的解决方案。

Answer 1

通常的方法是左连接:

select wd.wd, count(t.id)
from (select 1 as wd union all select 2 union all select 3 union all select 4 uion all
      select 5 union all select 6 union all select 7
     ) wd left join
     test t
     on wd.wd = weekday(t.date)
group by wd.wd
order by wd.wd;