How to create a SQL Server function to “join” multiple rows from a subquery into a single delimited field?
本问题已经有最佳答案，请猛点这里访问。
举例来说，假设我有两个表，如下所示：
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| VehicleID Name
1 Chuck
2 Larry
LocationID VehicleID City
1 1 NEW York
2 1 Seattle
3 1 Vancouver
4 2 Los Angeles
5 2 Houston |
我想编写一个查询来返回以下结果：
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| VehicleID Name Locations
1 Chuck NEW York, Seattle, Vancouver
2 Larry Los Angeles, Houston |
我知道这可以使用服务器端光标来完成，即：
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| DECLARE @VehicleID INT
DECLARE @VehicleName VARCHAR(100)
DECLARE @LocationCity VARCHAR(100)
DECLARE @Locations VARCHAR(4000)
DECLARE @Results TABLE
(
VehicleID INT
Name VARCHAR(100)
Locations VARCHAR(4000)
)
DECLARE VehiclesCursor CURSOR FOR
SELECT
[VehicleID]
, [Name]
FROM [Vehicles]
OPEN VehiclesCursor
FETCH NEXT FROM VehiclesCursor INTO
@VehicleID
, @VehicleName
WHILE @@FETCH_STATUS = 0
BEGIN
SET @Locations = ''
DECLARE LocationsCursor CURSOR FOR
SELECT
[City]
FROM [Locations]
WHERE [VehicleID] = @VehicleID
OPEN LocationsCursor
FETCH NEXT FROM LocationsCursor INTO
@LocationCity
WHILE @@FETCH_STATUS = 0
BEGIN
SET @Locations = @Locations + @LocationCity
FETCH NEXT FROM LocationsCursor INTO
@LocationCity
END
CLOSE LocationsCursor
DEALLOCATE LocationsCursor
INSERT INTO @Results (VehicleID, Name, Locations) SELECT @VehicleID, @Name, @Locations
END
CLOSE VehiclesCursor
DEALLOCATE VehiclesCursor
SELECT * FROM @Results |
但是，正如您所看到的，这需要大量的代码。我想要的是一个通用函数，它允许我执行如下操作：
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| SELECT VehicleID
, Name
, JOIN(SELECT City FROM Locations WHERE VehicleID = Vehicles.VehicleID, ', ') AS Locations
FROM Vehicles |
这有可能吗？或者类似的东西？
如果您使用的是SQL Server 2005，那么可以使用for xml path命令。
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| SELECT [VehicleID]
, [Name]
, (STUFF((SELECT CAST(', ' + [City] AS VARCHAR(MAX))
FROM [Location]
WHERE (VehicleID = Vehicle.VehicleID)
FOR XML PATH ('')), 1, 2, '')) AS Locations
FROM [Vehicle] |
它比使用光标容易得多，而且似乎工作得相当好。
请注意，Matt的代码将在字符串末尾产生一个额外的逗号；使用coalesce(或isnull)，如Lance文章中的链接所示，使用类似的方法，但不会给您留下额外的逗号来删除。为了完整起见，下面是来自sqlteam.com上Lance链接的相关代码：
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| DECLARE @EmployeeList VARCHAR(100)
SELECT @EmployeeList = COALESCE(@EmployeeList + ', ', '') +
CAST(EmpUniqueID AS VARCHAR(5))
FROM SalesCallsEmployees
WHERE SalCal_UniqueID = 1 |
我不认为有一种方法可以在一个查询中完成这项工作，但是您可以用一个临时变量来玩这样的把戏：
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| DECLARE @s VARCHAR(MAX)
SET @s = ''
SELECT @s = @s + City + ',' FROM Locations
SELECT @s |
它的代码绝对比浏览光标少，而且可能更高效。
In a single SQL query, without using the FOR XML clause.
A Common Table Expression is used to recursively concatenate the results.
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| -- rank locations by incrementing lexicographical order
WITH RankedLocations AS (
SELECT
VehicleID,
City,
ROW_NUMBER() OVER (
PARTITION BY VehicleID
ORDER BY City
) Rank
FROM
Locations
),
-- concatenate locations using a recursive query
-- (Common Table Expression)
Concatenations AS (
-- for each vehicle, select the first location
SELECT
VehicleID,
CONVERT(nvarchar(MAX), City) Cities,
Rank
FROM
RankedLocations
WHERE
Rank = 1
-- then incrementally concatenate with the next location
-- this will return intermediate concatenations that will be
-- filtered out later on
UNION ALL
SELECT
c.VehicleID,
(c.Cities + ', ' + l.City) Cities,
l.Rank
FROM
Concatenations c -- this is a recursion!
INNER JOIN RankedLocations l ON
l.VehicleID = c.VehicleID
AND l.Rank = c.Rank + 1
),
-- rank concatenation results by decrementing length
-- (rank 1 will always be for the longest concatenation)
RankedConcatenations AS (
SELECT
VehicleID,
Cities,
ROW_NUMBER() OVER (
PARTITION BY VehicleID
ORDER BY Rank DESC
) Rank
FROM
Concatenations
)
-- main query
SELECT
v.VehicleID,
v.Name,
c.Cities
FROM
Vehicles v
INNER JOIN RankedConcatenations c ON
c.VehicleID = v.VehicleID
AND c.Rank = 1 |
从我所看到的来看，如果您想像OP那样选择其他列(我猜大多数情况下都是这样)，那么FOR XML是唯一的方法。使用COALESCE(@var...不允许包含其他列。 更新：由于ProgrammingSolutions.net，有一种方法可以删除"尾随"逗号。通过将其设为前导逗号并使用mssql的STUFF函数，您可以将第一个字符(前导逗号)替换为空字符串，如下所示：
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| stuff(
(SELECT ',' + COLUMN
FROM TABLE
INNER WHERE INNER.Id = OUTER.Id
FOR xml path('')
), 1,1,'') AS VALUES |
在SQL Server 2005中
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| SELECT Stuff(
(SELECT N', ' + Name FROM Names FOR XML PATH(''),TYPE)
.value('text()[1]','nvarchar(max)'),1,2,N'') |
在SQL Server 2016中 可以使用for json语法 即
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| SELECT per.ID,
Emails = JSON_VALUE(
REPLACE(
(SELECT _ = em.Email FROM Email em WHERE em.Person = per.ID FOR JSON PATH)
,'
<div class="suo-content">[collapse title=""]<ul><li>我拿出了这一块：<wyn>TYPE ).value('text()[1]', 'nvarchar(MAX)')</wyn>，它仍然很管用…不知道该怎么办。</li><li>假设要解码XML，如果[City]具有类似字符的&；<>，则输出将变为&；amp；&；lt；&；gt；，如果您确定[City]没有这些特殊字符，则可以安全地删除它。——张士骏</li><li>+ 1。这个答案被低估了。您应该编辑它以提到这是唯一不会转义特殊字符(如&；<>等)的答案之一。此外，如果我们使用：<wyn>.value('.', 'nvarchar(MAX)')</wyn>，结果会不会相同？</li><li>嗨，宝达，结果是一样的，但是我测试的时候，使用"text()[1]"而不是"."时性能更好，没有什么区别。</li></ul>[/collapse]</div><p><center>[wp_ad_camp_2]</center></p><hr><P>以下代码适用于SQL Server 2000/2005/2008</P>[cc lang="sql"]CREATE FUNCTION fnConcatVehicleCities(@VehicleId SMALLINT)
RETURNS VARCHAR(1000) AS
BEGIN
DECLARE @csvCities VARCHAR(1000)
SELECT @csvCities = COALESCE(@csvCities + ', ', '') + COALESCE(City,'')
FROM Vehicles
WHERE VehicleId = @VehicleId
return @csvCities
END
-- //Once the User defined function is created then run the below sql
SELECT VehicleID
, dbo.fnConcatVehicleCities(VehicleId) AS Locations
FROM Vehicles
GROUP BY VehicleID |
我通过创建以下函数找到了一个解决方案：
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| CREATE FUNCTION [dbo].[JoinTexts]
(
@delimiter VARCHAR(20) ,
@whereClause VARCHAR(1)
)
RETURNS VARCHAR(MAX)
AS
BEGIN
DECLARE @Texts VARCHAR(MAX)
SELECT @Texts = COALESCE(@Texts + @delimiter, '') + T.Texto
FROM SomeTable AS T
WHERE T.SomeOtherColumn = @whereClause
RETURN @Texts
END
GO |
用途：
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| SELECT dbo.JoinTexts(' , ', 'Y') |
孟的回答对我不起作用，所以我对那个答案做了一些修改，让它起作用。希望这能帮助别人。使用SQL Server 2012:
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| SELECT [VehicleID]
, [Name]
, STUFF((SELECT DISTINCT ',' + CONVERT(VARCHAR,City)
FROM [Location]
WHERE (VehicleID = Vehicle.VehicleID)
FOR XML PATH ('')), 1, 2, '') AS Locations
FROM [Vehicle] |
对于其他答案，阅读答案的人必须了解车辆表，并创建车辆表和数据以测试解决方案。 下面是一个使用SQL Server"Information_schema.columns"表的示例。通过使用此解决方案，不需要创建表或添加数据。此示例为数据库中的所有表创建一个以逗号分隔的列名列表。
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| SELECT
TABLE_NAME
,STUFF((
SELECT ',' + Column_Name
FROM INFORMATION_SCHEMA.Columns COLUMNS
WHERE TABLES.Table_Name = COLUMNS.Table_Name
ORDER BY Column_Name
FOR XML PATH ('')), 1, 1, ''
)COLUMNS
FROM INFORMATION_SCHEMA.Columns TABLES
GROUP BY TABLE_NAME |
版本说明：对于此解决方案，必须使用兼容级别设置为90或更高的SQL Server 2005或更高版本。 有关创建用户定义聚合函数的第一个示例，该函数连接从表中的列中获取的一组字符串值，请参阅此msdn文章。 我谦虚的建议是去掉附加的逗号，这样您就可以使用自己的特别分隔符(如果有的话)。 参考实施例1的C版本：
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| CHANGE: this.intermediateResult.Append(VALUE.Value).Append(',');
TO: this.intermediateResult.Append(VALUE.Value); |
和
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| CHANGE: output = this.intermediateResult.ToString(0, this.intermediateResult.Length - 1);
TO: output = this.intermediateResult.ToString(); |
这样，当使用自定义聚合时，您可以选择使用自己的分隔符，或者完全不使用分隔符，例如：
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| SELECT dbo.CONCATENATE(column1 + '|') FROM table1 |
注意：请注意您试图在聚合中处理的数据量。如果尝试连接数千行或许多非常大的数据类型，则可能会出现.NET框架错误，说明"缓冲区不足"。
尝试这个查询
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| SELECT v.VehicleId, v.Name, ll.LocationList
FROM Vehicles v
LEFT JOIN
(SELECT
DISTINCT
VehicleId,
REPLACE(
REPLACE(
REPLACE(
(
SELECT City AS c
FROM Locations x
WHERE x.VehicleID = l.VehicleID FOR XML PATH('')
),
'</c><c>',', '
),
'<c>',''
),
'</c>', ''
) AS LocationList
FROM Locations l
) ll ON ll.VehicleId = v.VehicleId |
如果您运行的是SQL Server2005，那么可以编写一个自定义的clr聚合函数来处理这个问题。 C版本：
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| USING System;
USING System.Data;
USING System.Data.SqlClient;
USING System.Data.SqlTypes;
USING System.Text;
USING Microsoft.SqlServer.Server;
[Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedAggregate(Format.UserDefined,MaxByteSize=8000)]
public class CSV:IBinarySerialize
{
private StringBuilder RESULT;
public void Init() {
this.Result = NEW StringBuilder();
}
public void Accumulate(SqlString VALUE) {
IF (VALUE.IsNull) RETURN;
this.Result.Append(VALUE.Value).Append(",");
}
public void MERGE(CSV GROUP) {
this.Result.Append(GROUP.Result);
}
public SqlString Terminate() {
RETURN NEW SqlString(this.Result.ToString());
}
public void READ(System.IO.BinaryReader r) {
this.Result = NEW StringBuilder(r.ReadString());
}
public void WRITE(System.IO.BinaryWriter w) {
w.Write(this.Result.ToString());
}
} |
|
