将子查询中的多个结果组合成一个逗号分隔值-数据库问题

Combine multiple results in a subquery into a single comma-separated value(将子查询中的多个结果组合成一个逗号分隔值)

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问题描述

I've got two tables:

TableA
------
ID,
Name

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

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

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

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

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

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.

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

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

解决方案

1. Create the 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. Use in subquery:

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

3. If you are using stored procedure you can do like this:

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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