将子查询中的多个结果合并为一个以逗号分隔的值

2021-12-02 00:00:00 join tsql sql-server

我有两张桌子:

TableA
------
ID,
Name

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

关系是一行TableA - 许多TableB.

The relationship is one row of TableA - many of TableB.

现在，我想看到这样的结果:

Now, I want to see a result like this:

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):

This won't work (multiple results in a subquery):

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 次查询，其中 X 是 TableA 的结果数.

This is a trivial problem if I do the processing on the client side. But this will mean I will have to run X queries on every page, where X is the number of results of TableA.

请注意，我不能简单地执行 GROUP BY 或类似操作，因为它会为 TableA 的行返回多个结果.

Note that I can't simply do a GROUP BY or something similar, as it will return multiple results for rows of TableA.

我不确定使用 COALESCE 或类似方法的 UDF 是否可行?

I'm not sure if a UDF, utilizing COALESCE or something similar might work?

推荐答案

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.如果您使用的是存储过程，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

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