tsql左外部联接，在一行上显示多个记录

嗨，我有两个1-N关系的表，其中N最大为3。一个组至少有1个用户，最多有3个用户。我想用select查询在一行上显示组和所有可能的用户

组：

ID  Name
1   Group1
2   Group2

用户：

ID  Username IDGroup
1   User1    1
2   User2    2
3   User3    1
4   User4    1

结果（没有用户名显示为ok null或空字符串）：

你可以用

更新：

如果需要更多的字段，我会这样做

select T.IDGroup,
       T.GroupName,
       max(case when T.rn = 1 then T.Username end) as Username1,
       max(case when T.rn = 1 then T.Email end) as Email1,
       max(case when T.rn = 2 then T.Username end) as Username2,
       max(case when T.rn = 2 then T.Email end) as Email2,
       max(case when T.rn = 3 then T.Username end) as Username3,
       max(case when T.rn = 3 then T.Email end) as Email3
from (
     select G.ID as IDGroup,
            G.Name as GroupName,
            U.Username,
            U.Email,
            row_number() over(partition by G.ID order by U.Username) as rn
     from Groups as G
       left outer join Users as U
         on G.ID = U.IDGroup
     ) as T
group by T.IDGroup,
         T.GroupName

---

我还想提供这个答案，因为它也很好，而且在我看来，如果您想添加其他字段，它更灵活：

select  
        T.IDGroup
       ,T.GroupName
       ,[1] = max(case when rn = 1 then T.Username end)
       ,[2] = max(case when rn = 2 then T.Username end)
       ,[3] = max(case when rn = 3 then T.Username end)
from 
  (
  select G.ID as IDGroup,
         G.Name as GroupName,
         U.Username,
         row_number() over(partition by G.ID order by U.Username) as rn
  from Groups as G
    left outer join Users as U
      on G.ID = U.IDGroup
  ) as T
group by T.IDGroup, T.GroupName

---

我认为这是不可能的，因为结果中的列数将根据组中的用户数进行更改。为什么不使用简单的内部联接将其显示为每行的每个用户？否列的数量将始终相同（5），因为每个组的用户不超过3个。我需要这个布局是因为一个特定于业务的应用程序。我会给你回复的。但是你能告诉我一个ID组的最大ID数吗？谢谢！是的，每个组最多3个用户（如果可以显示此组中的前3个用户，而不考虑组中可能的用户数），我已提交了一个类似的。谢谢你，我以前从未听说过pivot，它是一个真正的救命软件！非常感谢！如果你不介意的话，我再问你一个问题。我在用户表中还有一个电子邮件字段。我如何使用pivot将其显示给用户名？@Arno2501好的，我看到您已经将其排序为+1。

select T.IDGroup,
       T.GroupName,
       max(case when T.rn = 1 then T.Username end) as Username1,
       max(case when T.rn = 1 then T.Email end) as Email1,
       max(case when T.rn = 2 then T.Username end) as Username2,
       max(case when T.rn = 2 then T.Email end) as Email2,
       max(case when T.rn = 3 then T.Username end) as Username3,
       max(case when T.rn = 3 then T.Email end) as Email3
from (
     select G.ID as IDGroup,
            G.Name as GroupName,
            U.Username,
            U.Email,
            row_number() over(partition by G.ID order by U.Username) as rn
     from Groups as G
       left outer join Users as U
         on G.ID = U.IDGroup
     ) as T
group by T.IDGroup,
         T.GroupName

select  
        T.IDGroup
       ,T.GroupName
       ,[1] = max(case when rn = 1 then T.Username end)
       ,[2] = max(case when rn = 2 then T.Username end)
       ,[3] = max(case when rn = 3 then T.Username end)
from 
  (
  select G.ID as IDGroup,
         G.Name as GroupName,
         U.Username,
         row_number() over(partition by G.ID order by U.Username) as rn
  from Groups as G
    left outer join Users as U
      on G.ID = U.IDGroup
  ) as T
group by T.IDGroup, T.GroupName