How to create a SQL Server function to "join" multiple rows from a subquery into a single delimited field? [duplicate]

Question

If you're using SQL Server 2005, you could use the FOR XML PATH command.

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]

It's a lot easier than using a cursor, and seems to work fairly well.

Update

For anyone still using this method with newer versions of SQL Server, there is another way of doing it which is a bit easier and more performant using the STRING_AGG method that has been available since SQL Server 2017.

SELECT  [VehicleID]
       ,[Name]
       ,(SELECT STRING_AGG([City], ', ')
         FROM [Location]
         WHERE VehicleID = V.VehicleID) AS Locations
FROM   [Vehicle] V

This also allows a different separator to be specified as the second parameter, providing a little more flexibility over the former method.

Note that Matt's code will result in an extra comma at the end of the string; using COALESCE (or ISNULL for that matter) as shown in the link in Lance's post uses a similar method but doesn't leave you with an extra comma to remove. For the sake of completeness, here's the relevant code from Lance's link on sqlteam.com:

DECLARE @EmployeeList varchar(100)
SELECT @EmployeeList = COALESCE(@EmployeeList + ', ', '') + 
    CAST(EmpUniqueID AS varchar(5))
FROM SalesCallsEmployees
WHERE SalCal_UniqueID = 1

I don't belive there's a way to do it within one query, but you can play tricks like this with a temporary variable:

declare @s varchar(max)
set @s = ''
select @s = @s + City + ',' from Locations

select @s

It's definitely less code than walking over a cursor, and probably more efficient.

In a single SQL query, without using the FOR XML clause.
A Common Table Expression is used to recursively concatenate the results.

-- 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

From what I can see FOR XML (as posted earlier) is the only way to do it if you want to also select other columns (which I'd guess most would) as the OP does. Using COALESCE(@var... does not allow inclusion of other columns.

Update: Thanks to programmingsolutions.net there is a way to remove the "trailing" comma to. By making it into a leading comma and using the STUFF function of MSSQL you can replace the first character (leading comma) with an empty string as below:

stuff(
    (select ',' + Column 
     from Table
         inner where inner.Id = outer.Id 
     for xml path('')
), 1,1,'') as Values

In SQL Server 2005

SELECT Stuff(
  (SELECT N', ' + Name FROM Names FOR XML PATH(''),TYPE)
  .value('text()[1]','nvarchar(max)'),1,2,N'')

---

In SQL Server 2016

you can use the FOR JSON syntax

i.e.

SELECT per.ID,
Emails = JSON_VALUE(
   REPLACE(
     (SELECT _ = em.Email FROM Email em WHERE em.Person = per.ID FOR JSON PATH)
    ,'"},{"_":"',', '),'$[0]._'
) 
FROM Person per

And the result will become

Id  Emails
1   [email protected]
2   NULL
3   [email protected], [email protected]

This will work even your data contains invalid XML characters

the '"},{"":"' is safe because if you data contain '"},{"":"', it will be escaped to "},{\"_\":\"

You can replace ', ' with any string separator

---

And in SQL Server 2017, Azure SQL Database

You can use the new STRING_AGG function

The below code will work for 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