SQL Count：如何从其他列的相同值中计算一列的值

Question

抱歉，标题。

示例：我有这张桌子（tblTry）：

id | Name | Color
____________________
1  | XYZ  | Black
2  | XYZ  | Black
3  | ASD  | Red
4  | ASD  | White
5  | ASD  | White

这是我想要的输出：

Name | Black | Red | White
__________________________
XYZ  | 2     |  0  |  0
ASD  | 0     |  1  |  2

我有这个SQL，但它给了我不同的输出：

select distinct
Name,
(select count(*) from tblTry where Color= 'Black') as Black,
(select count(*) from tblTry where Color= 'Red') as Red,
(select count(*) from tblTry where Color= 'White') as White,
from tblTry
group by Name

上面的sql输出：

__________________________
Name | Black | Red | White
__________________________
XYZ  | 2     |  1  |  2
ASD  | 2     |  1  |  2

谁能帮我？

谢谢

Answer 1

这是一个枢纽，有几种解决方案。 跨数据库的普遍做法是使用条件聚合：

select name,
       sum(case when Color = 'Black' then 1 else 0 end) as Black,
       sum(case when Color = 'Red' then 1 else 0 end) as Red,
       sum(case when Color = 'White' then 1 else 0 end) as White
from tblTry
group by name;

您的查询的问题是计数需要与每一行相关。 您可以使用其他where条件来执行此操作：

(select count(*) from tblTry t2 where t2.Color= 'Black' and t2.name = tblTry.name) as Black,