来自同一表的GROUP BY和LEFT JOIN以及COUNT个

Question

要根据年份，类型和订单按月计数

这是我的user table ：

id  type  uname      date_created
1    fac   a      2015-12-28 17:11:19
2    cs    b      2015-12-23 19:09:51
3    cs    c      2015-12-23 19:09:21
4    stu   d      2015-12-31 18:12:41
5    fac   e      2015-11-11 00:00:00
6    fac   f      2015-10-07 00:00:00

预期结果：

fac  stu  cs
 1    1    2 // month january
 1    0    0 // month octomber
 1    0    0 // month november

我试过的是：

SELECT count(u1.id) as fac, count(u2.id) as stu, count(u3.id) as cs
FROM user u
left join user u1 ON u1.faculty = 'yes' AND YEAR(u1.date_created) = 2015
left join user u2 ON u2.faculty = 'no' AND YEAR(u2.date_created) = 2015
left join user u3 ON u3.faculty = 'club_student' AND YEAR(u3.date_created) = 2015
GROUP BY MONTH(u.date_created) ORDER BY MONTH(u.date_created)

给我错误的结果，例如：

fac  stu  cs
6    6    6 
6    6    6 
24   24   24

Answer 1

您需要一个PIVOT命令将行转置为列。 实际上，MySQL不支持这种操作，因此我们需要使用CASE WHEN手动进行操作（请参见SQLFiddle ）：

select 
  month(date_created) as month,
  count(case when faculty = 'yes' THEN 1 END) as fac,
  count(case when faculty = 'no'  THEN 1 END) as stu,
  count(case when faculty = 'club_student' THEN 1 END) as cs
from user
where 1=1
  and date_created >= STR_TO_DATE('01-01-2015','%d-%m-%Y')
  and date_created <  STR_TO_DATE('01-01-2016','%d-%m-%Y')
group by month(date_created)
order by month(date_created)

实际上，您的联接语法没有意义...您需要进行三次联接以计算每个表中的id ...截至左联接时，只要存在联接匹配项，这些表中的数据就会返回，在不匹配的情况下返回null ...在COUNT聚合函数中不计算null值，因此您的联接语法可以简化为INNER联接语法。 但是，您甚至不需要INNER FROM ，因为您正在FROM相同的表，并且实际上您并不将FROM表中的任何列与JOIN表相关联。

这样，您定义的过滤器可以简化为WHERE过滤器和CASE WHEN过滤器（如我所建议的）。

Answer 2

试试count(distinct) ：

SELECT count(distinct u1.id) as fac, count(distinct u2.id) as stu,
       count(distinct u3.id) as cs
FROM user u left join
     user u1
     ON u1.faculty = 'yes' AND YEAR(u1.date_created) = 2015 left join
     user u2
     ON u2.faculty = 'no' AND YEAR(u2.date_created) = 2015 left join
     user u3
     ON u3.faculty = 'club_student' AND YEAR(u3.date_created) = 2015
GROUP BY MONTH(u.date_created) ORDER BY MONTH(u.date_created)