An SQL script that uses recursive CTE to do ordered cumulative string concatenation

cumulative_string_concatenation.sql

-- ============================================================================
-- Filename:     cumulative_string_concatenation.sql
--
-- Author:       Mattias Spångmyr
-- Create date:  2020-04-16
-- Description:  Uses a recursive common table expression (recursive CTE) to
--               cumulatively concatenace string values that have a defined
--               order and share a common identifier.
--
-- SQL Server
-- version:      MS SQL Server 2017
--
-- Copyright:    GPL v3 or later.
--               This program is free software: you can redistribute it and/or
--               modify it under the terms of the GNU General Public License as
--               published by the Free Software Foundation, either version 3 of
--               the License, or (at your option) any later version.
--
--               This program is distributed in the hope that it will be
--               useful, but WITHOUT ANY WARRANTY; without even the implied
--               warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
--               PURPOSE.  See the GNU General Public License for more details.
--
--               You should have received a copy of the GNU General Public
--               License along with this program.  If not, see
--               https://www.gnu.org/licenses/.
--
-- Docs
-- reference:    https://docs.microsoft.com/en-us/sql/t-sql/queries/with-common-table-expression-transact-sql
--
-- ============================================================================

DECLARE @TEMP AS TABLE (
   id int null,
   val varchar(100) null,
   i int null
)
INSERT INTO @TEMP
VALUES (1, '2', 1), (1, '3', 2), (1, '4', 3), (1, '5', 4),
   (2, '1', 1), (2, '3', 2), (2, '4', 3), (2, '5', 4),
   (3, '2', 1), (3, '4', 2), (3, '1', 3), (3, '5', 4),
   (4, '3', 1), (4, '5', 2), (4, '2', 3), (4, '1', 4),
   (5, '4', 1), (5, '3', 2), (5, '2', 3), (5, '1', 4);

WITH gr (n, id, val, i) AS
(
SELECT 1
   , id
   , val
   , i
FROM @TEMP
WHERE i = 1
UNION ALL
SELECT gr.n + 1
   , temp.id
   , CAST(CONCAT_WS(',', gr.val, temp.val) AS varchar(100))
   , temp.i
FROM @TEMP temp
INNER JOIN gr ON temp.id = gr.id AND temp.i = (n + 1)
)
SELECT id, val, i
FROM gr
ORDER BY id, i
OPTION (MAXRECURSION 1000)