SQL Server 2022

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CREATE TABLE YourTable(CTransaction VARCHAR(3), CQuantity int, CItemCode VARCHAR(5), CUOM VARCHAR(10));

INSERT YourTable
VALUES

( '001',  2,  '0011', 'PACK'), 
( '001',  5,  '0011', 'PACK'), 
( '001',  1,  '0011', 'PACK'), 
( '002',  3,  '0011', 'PACK'), 
( '002',  3,  '0011', 'PACK')
5 rows affected

WITH Tgt AS
(
SELECT *, 
        SumCQuantity = SUM(CQuantity) OVER (PARTITION BY CTransaction, CItemCode, CUOM ),
        rn = ROW_NUMBER() OVER (PARTITION BY CTransaction, CItemCode, CUOM  ORDER BY CTransaction) 
FROM YourTable
) 
Merge Tgt with (HOLDLOCK)
USING (VALUES(1)) src(rn) ON Tgt.rn = 1
WHEN MATCHED AND CQuantity IS DISTINCT FROM SumCQuantity THEN 
    UPDATE SET CQuantity = SumCQuantity 
WHEN NOT MATCHED BY SOURCE THEN 
    DELETE;
5 rows affected

SELECT *
FROM YourTable


CTransaction
CQuantity
CItemCode
CUOM


001
8
0011
PACK

002
6
0011
PACK