SQL Server 2017

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CREATE TABLE test (Product INT, Qty INT)
INSERT INTO test VALUES (1, 21), (2, 32)
2 rows affected

SELECT * FROM test


Product
Qty


1
21

2
32


WITH cte0 AS ( SELECT 10 chunk 
             ),
     cte1 AS ( SELECT 1 + MAX(qty / chunk) chunks
               FROM test, cte0 
             ),
     cte2 AS ( SELECT 1 num
               UNION ALL
               SELECT num+1
               FROM cte1, cte2
               WHERE num < chunks
             ),
     cte3 AS ( SELECT num, Product, CASE WHEN qty > num * chunk 
                                         THEN chunk 
                                         ELSE qty - (num - 1) * chunk 
                                         END qty
               FROM test, cte0, cte2
               WHERE CASE WHEN qty > num * chunk 
                          THEN chunk 
                          ELSE qty - (num - 1) * chunk 
                          END > 0
             )
MERGE test
USING cte3
ON (test.Product = cte3.Product AND cte3.num =1)
WHEN MATCHED THEN 
    UPDATE SET Qty = cte3.qty
WHEN NOT MATCHED THEN
    INSERT (Product, Qty)
    VALUES (cte3.Product, cte3.qty);
7 rows affected

SELECT * FROM test ORDER BY Product, Qty DESC


Product
Qty


1
10

1
10

1
1

2
10

2
10

2
10

2
2