SQL Server - 将行连接到逗号分隔列表中

Question

假设我有一个临时表,如下所示:

+----+------+
| Id | Value|
+----+------+    
| 1  |   1  |
| 1  |   2  |
| 1  |   3  |
| 2  |   1  |
| 2  |   2  |
+----+------+

我希望我的桌子像这样:

    +----+----------+
    | Id | ValueList|
    +----+----------+    
    | 1  |   1,2,3  |
    | 2  |   1,2    | 
    +----+----------+

所以基本上我需要将我的值分组为逗号分隔列表.我已经尝试过以下方法:

SELECT Id, STUFF((SELECT ',' + CAST(VALUE AS varchar) FROM @MyTable FOR XML PATH('')), 1 ,1, '') AS ValueList
FROM @MyTable
GROUP BY Id

但我得到的结论是:

        +----+---------------------+
        | Id |      ValueList      |
        +----+---------------------+    
        | 1  |   1,1,1,1,1,1,...   |
        +----+---------------------+

我找不到我做错了什么.有人可以帮助这个查询吗？还是指向正确的方向？谢谢.

Answer 1

您缺少子查询中的条件.

SELECT t2.Id, STUFF((SELECT ',' + CAST(VALUE AS varchar) FROM @MyTable t1  where t1.Id =t2.ID FOR XML PATH('')), 1 ,1, '') AS ValueList
FROM @MyTable t2
GROUP BY t2.Id

演示

Answer 2

为此，还可以将 FOR XML 与 CROSS APPLY（或 OUTER APPLY）结合使用。

示例片段：

declare @T table (id int, value int);
insert into @T values (1,1),(1,2),(1,3),(2,1),(2,2);

select id, stuff(x.list,1,1,'') as list
from (select distinct id from @T) as t
cross apply (
 select concat(',',t2.value)
 from @T t2
 where t2.id = t.id 
 for xml path('')
) x(list)
order by id;

结果：

id  list
--  -----
1   1,2,3
2   1,2

从 MS Sql Server 2017 开始，可以改用STRING_AGG。

select id, string_agg(value,',') as list
from Yourtable t
group by id;

Answer 3

一个替代使用GROUP BY的Id是使用不同的选择:

SELECT DISTINCT
    Id,
    STUFF((SELECT ',' + CAST(t2.VALUE AS varchar)
           FROM @MyTable t2
           WHERE t2.Id = t1.Id
           FOR XML PATH('')), 1 ,1, '') AS ValueList
FROM @MyTable t1

演示