Custom aggregate function (concat) in SQL Server - sql

Question

Question: I want to write a custom aggregate function that concatenates string on group by.
So that I can do a
SELECT SUM(FIELD1) as f1, MYCONCAT(FIELD2) as f2
FROM TABLE_XY
GROUP BY FIELD1, FIELD2
All I find is SQL CRL aggregate functions, but I need SQL, without CLR.
Edit:1
The query should look like this:
SELECT SUM(FIELD1) as f1, MYCONCAT(FIELD2) as f2
FROM TABLE_XY
GROUP BY FIELD0
Edit 2:
It is true that it isn't possible without CLR.
However, the subselect answer by astander can be modified so it doesn't XML-encode special characters.
The subtle change for this is to add this after "FOR XML PATH":
,
TYPE
).value('.[1]', 'nvarchar(MAX)')
Here a few examples
DECLARE #tT table([A] varchar(200), [B] varchar(200));
INSERT INTO #tT VALUES ('T_A', 'C_A');
INSERT INTO #tT VALUES ('T_A', 'C_B');
INSERT INTO #tT VALUES ('T_B', 'C_A');
INSERT INTO #tT VALUES ('T_C', 'C_A');
INSERT INTO #tT VALUES ('T_C', 'C_B');
INSERT INTO #tT VALUES ('T_C', 'C_C');
SELECT
A AS [A]
,
(
STUFF
(
(
SELECT DISTINCT
', ' + tempT.B AS wtf
FROM #tT AS tempT
WHERE (1=1)
--AND tempT.TT_Status = 1
AND tempT.A = myT.A
ORDER BY wtf
FOR XML PATH, TYPE
).value('.[1]', 'nvarchar(MAX)')
, 1, 2, ''
)
) AS [B]
FROM #tT AS myT
GROUP BY A
SELECT
(
SELECT
',äöü<>' + RM_NR AS [text()]
FROM T_Room
WHERE RM_Status = 1
ORDER BY RM_NR
FOR XML PATH('')
) AS XmlEncodedNoNothing
,
SUBSTRING
(
(
SELECT
',äöü<>' + RM_NR AS [data()]
FROM T_Room
WHERE RM_Status = 1
ORDER BY RM_NR
FOR XML PATH('')
)
,2
,10000
) AS XmlEncodedSubstring
,
(
STUFF
(
(
SELECT ',äöü<>' + RM_NR + CHAR(10)
FROM T_Room
WHERE RM_Status = 1
ORDER BY RM_NR
FOR XML PATH, TYPE
).value('.[1]', 'nvarchar(MAX)')
, 1, 1, ''
)
) AS XmlDecodedStuffInsteadSubstring

Answer 1

You cannot write custom aggregates outside of the CLR.
The only type of functions you can write in pure T-SQL are scalar and table valued functions.
Compare the pages for CREATE AGGREGATE, which only lists CLR style options, with CREATE FUNCTION, which shows T-SQL and CLR options.

Answer 2

Have a look at something like. This is not an aggregate function. If you wish to implement your own aggregate function, it will have to be CLR...
DECLARE #Table TABLE(
ID INT,
Val VARCHAR(50)
)
INSERT INTO #Table (ID,Val) SELECT 1, 'A'
INSERT INTO #Table (ID,Val) SELECT 1, 'B'
INSERT INTO #Table (ID,Val) SELECT 1, 'C'
INSERT INTO #Table (ID,Val) SELECT 2, 'B'
INSERT INTO #Table (ID,Val) SELECT 2, 'C'
--Concat
SELECT t.ID,
SUM(t.ID),
stuff(
(
select ',' + t1.Val
from #Table t1
where t1.ID = t.ID
order by t1.Val
for xml path('')
),1,1,'') Concats
FROM #Table t
GROUP BY t.ID

Answer 3

Starting from 2017 there is built-in concatenate aggregate function STRING_AGG :)
https://learn.microsoft.com/en-us/sql/t-sql/functions/string-agg-transact-sql?view=sql-server-2017

Answer 4

Found this link around concatenation which covers methods like
Concatenating values when the number of items are not known
Recursive CTE method
The blackbox XML methods
Using Common Language Runtime
Scalar UDF with recursion
Table valued UDF with a WHILE loop
Dynamic SQL
The Cursor approach
Non-reliable approaches
Scalar UDF with t-SQL update extension
Scalar UDF with variable concatenation in SELECT
Though it doesn't cover aggerate functions there may be some use around concatenation in there to help you with your problem.

Answer 5

This solution works with no need of deploy from Visual studio or dll file in server.
Copy-Paste and it Work!
https://github.com/orlando-colamatteo/ms-sql-server-group-concat-sqlclr
dbo.GROUP_CONCAT(VALUE )
dbo.GROUP_CONCAT_D(VALUE ), DELIMITER )
dbo.GROUP_CONCAT_DS(VALUE , DELIMITER , SORT_ORDER )
dbo.GROUP_CONCAT_S(VALUE , SORT_ORDER )

Answer 6

You could do something like what I have done below to create a custom aggregate concatenation function in pure T-SQL. Obviously I have gone with a hard coded table name and group by column but it should illustrate the approach. There is probably some way to make this a truly generic function using dynamic TSQL constructed from input parameters.
/*
User defined function to help perform concatenations as an aggregate function
Based on AdventureWorks2008R2 SalesOrderDetail table
*/
--select * from sales.SalesOrderDetail
IF EXISTS (SELECT *
FROM sysobjects
WHERE name = N'fnConcatenate')
DROP FUNCTION fnConcatenate
GO
CREATE FUNCTION fnConcatenate
(
#GroupByValue int
)
returnS varchar(8000)
as
BEGIN
DECLARE #SqlString varchar(8000)
Declare #TempStore varchar(25)
select #SqlString =''
Declare #MyCursor as Cursor
SET #MyCursor = CURSOR FAST_FORWARD
FOR
Select ProductID
From sales.SalesOrderDetail where SalesOrderID = #GroupByValue
order by SalesOrderDetailID asc
OPEN #MyCursor
FETCH NEXT FROM #MyCursor
INTO #TempStore
WHILE ##FETCH_STATUS = 0
BEGIN
select #SqlString = ltrim(rtrim(#TempStore )) +',' + ltrim(rtrim(#SqlString))
FETCH NEXT FROM #MyCursor INTO #TempStore
END
CLOSE #MyCursor
DEALLOCATE #MyCursor
RETURN #SqlString
END
GO
select SalesOrderID, Sum(OrderQty), COUNT(*) as DetailCount , dbo.fnConcatenate(salesOrderID) as ConCatenatedProductList
from sales.SalesOrderDetail
where salesOrderID= 56805
group by SalesOrderID