将子查询中的多个结果合并为一个逗号分隔值

Question

我有两张桌子:

TableA
------
ID,
Name

TableB
------
ID,
SomeColumn,
TableA_ID (FK for TableA)

这种关系是一排TableA- 许多TableB.

现在,我希望看到这样的结果:

ID     Name      SomeColumn

1.     ABC       X, Y, Z (these are three different rows)
2.     MNO       R, S

这不起作用(子查询中有多个结果):

SELECT ID,
       Name, 
       (SELECT SomeColumn FROM TableB WHERE F_ID=TableA.ID)
FROM TableA

如果我在客户端进行处理,这是一个微不足道的问题.但这意味着我必须在每个页面上运行X查询,其中X是结果的数量TableA.

请注意,我不能简单地执行GROUP BY或类似的操作,因为它将为行返回多个结果TableA.

我不确定使用COALESCE或类似东西的UDF是否可行？

Answer 1

即使这样也会达到目的

样本数据

declare @t table(id int, name varchar(20),somecolumn varchar(MAX))
insert into @t
    select 1,'ABC','X' union all
    select 1,'ABC','Y' union all
    select 1,'ABC','Z' union all
    select 2,'MNO','R' union all
    select 2,'MNO','S'

查询:

SELECT ID,Name,
    STUFF((SELECT ',' + CAST(T2.SomeColumn AS VARCHAR(MAX))
     FROM @T T2 WHERE T1.id = T2.id AND T1.name = T2.name
     FOR XML PATH('')),1,1,'') SOMECOLUMN
FROM @T T1
GROUP BY id,Name

输出:

ID  Name    SomeColumn
1   ABC     X,Y,Z
2   MNO     R,S

Answer 2

1.创建UDF:

CREATE FUNCTION CombineValues
(
    @FK_ID INT -- The foreign key from TableA which is used 
               -- to fetch corresponding records
)
RETURNS VARCHAR(8000)
AS
BEGIN
DECLARE @SomeColumnList VARCHAR(8000);

SELECT @SomeColumnList =
    COALESCE(@SomeColumnList + ', ', '') + CAST(SomeColumn AS varchar(20)) 
FROM TableB C
WHERE C.FK_ID = @FK_ID;

RETURN 
(
    SELECT @SomeColumnList
)
END

2.在子查询中使用:

SELECT ID, Name, dbo.CombineValues(FK_ID) FROM TableA

3.如果您使用存储过程,您可以这样做:

CREATE PROCEDURE GetCombinedValues
 @FK_ID int
As
BEGIN
DECLARE @SomeColumnList VARCHAR(800)
SELECT @SomeColumnList =
    COALESCE(@SomeColumnList + ', ', '') + CAST(SomeColumn AS varchar(20)) 
FROM TableB
WHERE FK_ID = @FK_ID 

Select *, @SomeColumnList as SelectedIds
    FROM 
        TableA
    WHERE 
        FK_ID = @FK_ID 
END

Answer 3

我认为你与COALESCE走在正确的轨道上.有关构建逗号分隔字符串的示例,请参见此处:

http://www.sqlteam.com/article/using-coalesce-to-build-comma-delimited-string

Answer 4

在MySQL中有一个group_concat函数,它将返回您要求的内容.

SELECT TableA.ID, TableA.Name, group_concat(TableB.SomeColumn) 
as SomColumnGroup FROM TableA LEFT JOIN TableB ON 
TableB.TableA_ID = TableA.ID