SQL 使用多列進行不同計數的查詢

Question

SQL服務器中有下表：

create table [dbo].[stats](
    [customerid] [int] NOT NULL,
    [username] [nvarchar](256) NOT NULL,
    [source] [int] NOT NULL,
    [destination] [int] NOT NULL,
    [date] [smalldatetime] NOT NULL
) 

填充數據：

customerid  username    source  destination date
1           user1         1        1        2022-05-06 00:00:00
1           user2         2        1        2022-05-06 00:00:00
1           user21        1        2        2022-05-06 00:00:00

有沒有一種方法可以創建一個查詢來顯示有多少唯一用戶使用了每個 customerid 的每種類型的源和目標？

使用上面的示例，我想要以下 output：

customerid source1 source2 destination1 destination2
1            2        1         2           1 

偽查詢：

select customerid,
    count(distinct(username + source)) where source = 1) as source1,
    count(distinct(username + source)) where source = 2) as source2,
    count(distinct(username + destination)) where destination = 1) as destination1,
    count(distinct(username + destination)) where destination = 2) as destination2,
from stats
group by customerid

Answer 1

你可以用例

select 
  customerid,
  count(distinct(case 
                 when source = 1
                 then username
                 End)) as source1,
    count(distinct(case
                 when source = 2
                 Then username
                 End )) as source2,
    count(distinct(case 
                 when destination = 1
                 Then username
                 End)) as destination1,
    count(distinct(case
                 when destination = 2
                 Then username
                 End)) as destination2,
from stats
group by customerid;

Answer 2

select userID, source, destination, count(*)
from stats
group by userID, source, destination

Answer 3

看起來你只需要一個條件計數

select customerid, 
    Count(case when source=1 then 1 end) Source1,
    Count(case when source=2 then 1 end) Source2,
    Count(case when destination=1 then 1 end) destination1,
    Count(case when destination=2 then 1 end) destination2
from stats
group by customerid;