Question

希望按年份按年份，类型和顺序计算

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

我尝试的是：

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，如：

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

Answer 1

您需要的是PIVOT命令，用于将行转置为列。实际上，MySQL不支持这种操作，因此我们需要使用CASE WHEN手动执行此操作（请参阅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，你的连接语法没有意义...你连接三次来计算每个表中的id ...从左连接开始，这些表中的数据只有在有这些表时才会返回连接匹配，如果不匹配则返回null ... COUNT聚合函数中不计算空值，因此您的连接语法可以简化为INNER连接语法。但是，当您加入相同的表时，甚至不需要INNER连接，并且实际上并未将FROM表中的任何列与JOIN表关联起来。

这样，您定义的过滤器可以缩减为WHERE过滤器和CASE WHEN过滤器。

Answer 2

尝试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)

GROUP BY和LEFT JOIN与来自同一个表的COUNT

2 个答案: