为了说明这一点,假设我有如下两个表:

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

我想写一个查询返回以下结果:

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游标完成,即:

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

然而,正如您所看到的,这需要大量的代码。我想要的是一个泛型函数,允许我做这样的事情:

SELECT VehicleID
     , Name
     , JOIN(SELECT City FROM Locations WHERE VehicleID = Vehicles.VehicleID, ', ') AS Locations
FROM Vehicles

这可能吗?或者类似的东西?


当前回答

在单个SQL查询中,不使用FOR XML子句。 公共表表达式用于递归地连接结果。

-- 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

其他回答

如果你正在运行SQL Server 2005,你可以编写一个自定义CLR聚合函数来处理这个问题。

c#版本:

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());
    }
}

下面的代码将适用于Sql Server 2000/2005/2008

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

试试这个查询

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

版本注意:此解决方案必须使用SQL Server 2005或更高版本,并将兼容性级别设置为90或更高版本。

请参阅这篇MSDN文章,了解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获取的一组字符串值。

我的建议是去掉附加的逗号,这样您就可以使用自己的特殊分隔符(如果有的话)。

参考示例1的c#版本:

change:  this.intermediateResult.Append(value.Value).Append(',');
    to:  this.intermediateResult.Append(value.Value);

And

change:  output = this.intermediateResult.ToString(0, this.intermediateResult.Length - 1);
    to:  output = this.intermediateResult.ToString();

这样,当你使用你的自定义聚合时,你可以选择使用你自己的分隔符,或者根本不使用,例如:

SELECT dbo.CONCATENATE(column1 + '|') from table1

注意:要注意您试图在聚合中处理的数据量。如果你试图连接数千行或许多非常大的数据类型,你可能会得到一个。net Framework错误,说明“[t]他缓冲区不足。”

妈妈的答案对我没用,所以我对答案做了一些修改,让它起作用。希望这能帮助到一些人。 使用SQL Server 2012:

SELECT [VehicleID]
     , [Name]
     , STUFF((SELECT DISTINCT ',' + CONVERT(VARCHAR,City) 
         FROM [Location] 
         WHERE (VehicleID = Vehicle.VehicleID) 
         FOR XML PATH ('')), 1, 2, '') AS Locations
FROM [Vehicle]