在 MySQL / SQL 中对同一行进行条件计数

Question

假设我有一张桌子，Foo，看起来像这样：

ID  | Name  | Gender  | Team
1   | Bob   | Male    | A
2   | Amy   | Female  | A
3   | Cat   | Female  | B
4   | Dave  | Male    | B
5   | Evan  | Male    | B

如果我想获得每个团队同一行的男性和女性人数列表，我该怎么做？

我知道我可以做到SELECT COUNT(Name) as "#", Team, Gender FROM foo GROUP BY Team, Gender，这对于大多数用途来说都很好。

但这会让我每个团队有 2 行，如下所示，这可能会很痛苦。

#  Team Gender
1 | A | Male
1 | A | Female
1 | B | Female
2 | B | Male

如何构造查询以使它们出现在同一行上？

即，

Team | Males | Females
A    |   1   | 1
B    |   2   | 1

Answer 1

select team,
 SUM(case when gender='Male' then 1 else 0 end) Male,
 SUM(case when gender='Female' then 1 else 0 end) Female
from tbl
group by team

为了评论

现在想象一下，每一行都有一个与之关联的任意数字分数，在分数列中。我将如何获得“男性积分”和“女性积分”？那会是SUM（当gender =“male”然后选择点其他0结束的情况）“男性点”

你已经接近了。答案是

select team,
 SUM(case when gender='Male' then 1 else 0 end) Male,
 SUM(case when gender='Male' then points else 0 end) `Male Points`,
 SUM(case when gender='Female' then 1 else 0 end) Female,
 SUM(case when gender='Female' then points else 0 end) `Female Points`
from tbl
group by team

Answer 2

我正在寻找的模式是 Self-Join；在我看来，语法和逻辑比 CASE 模式更优雅。

具体来说，

SELECT Males.Team, COUNT(Males.id) as "Males", SUM(Males.Points) as "Male Points", 
       COUNT(Females.id) as "Females", SUM(Females.Points) as "Female Points" 
FROM scores as Males 
LEFT JOIN scores as Females ON Males.Team=Females.Team AND Females.Gender="Female" 
WHERE Males.Gender="Male" 
GROUP BY Team

我想要在不同列中的分组被拆分为同一个表的自己的副本，而不是 case 语句。然后，您将男性玩家表与团队中的女性玩家表一起加入，然后按团队分组。