Question

在对UserID执行DISTINCT之后，我正在尝试计算女性用户数和男性用户数

SELECT
COUNT(IF("Gender" = 'female', 1, NULL)) as Ufemale,
COUNT(IF("Gender" = 'male', 1, NULL)) as Umale
FROM (SELECT DISTINCT UserID FROM user_stats where Year='2019' and Account='P') as UID

如果我执行

SELECT DISTINCT UserID FROM user_stats where Year='2019' and Account='P'

它返回唯一的用户ID。但是，如果我将其与性别计数部分结合使用，它将返回零。

这是值的样子

UserID            |  Gender
-----------------------------
2018359084885123  |  male
1925823664195671  |  female
2033134076795519  |
2122445674469149  |  female
2315129265210413  |  female
2018359084885123  |  male
2122445674469149  |  female

目的是展示

Ufemale  |  Umale
-------------------
   3     |    1

Answer 1

尝试一下：

表格

drop table if exists test;

create table test (userid bigint, gender char(6));

insert into test values
(2018359084885123,'male')
,(1925823664195671,'female')
,(2033134076795519, null)
,(2122445674469149,'female')
,(2315129265210413,'female')
,(2018359084885123,'male')
,(2122445674469149,'female');

查询

select
    sum(case when gender = 'female' then 1 else 0 end) as ufemale,
    sum(case when gender = 'male' then 1 else 0 end) as umale
from
(select distinct userid, gender from test) x

结果

ufemale umale
3       1

示例：https://rextester.com/JLQ19855

Answer 2

子查询的结果中没有Gender列：

SELECT DISTINCT UserID FROM user_stats where Year='2019' and Account='P'

您可能会想到。
您正在做的是比较两个字符串："Gender"和'female'，它们显然不相等，因此：

IF("Gender" = 'female', 1, NULL)

返回NULL，而COUNT(NULL)返回0。
Gender列在哪里？
是在user_stats表中还是像Users这样的其他表中？
如果它位于user_stats中，则需要这样编写查询：

select
  sum(t.gender = 'female') ufemale,
  sum(t.gender = 'male') umale
from (
  select distinct userid, gender 
  from user_stats 
  where Year='2019' and Account='P'
) t

如果它在users表中，则需要首先加入：

select
  sum(u.gender = 'female') ufemale,
  sum(u.gender = 'male') umale
from (
  select distinct userid, gender 
  from user_stats 
  where Year='2019' and Account='P'
) t inner join users u
on u.userid = t.userid

用IF FROM DISTINCT的COUNT值MySQL

2 个答案: