UPDATE		t
SET		t.Accum = t.Accum + s.Value
FROM		tblTarget AS t
INNER JOIN	(
			SELECT	DefinitionDate,
				BranchID,
				CommodityID,
				ROW_NUMBER() OVER (PARTITION BY DefinitionDate, BranchID, CommodityID ORDER BY DefinitionDate) AS recID,
				Value
			FROM	tblSource
		) AS s ON s.DefinitionDate = t.DefinitionDate
			AND s.BranchID = t.BranchID
			AND s.CommodityID = t.CommodityID
WHERE		s.recID = 1

INSERT	tblTarget
	(
		DefinitionDate,
		BranchID,
		CommodityID,
		Value
	)
SELECT	s.DefinitionDate,
	s.BranchID,
	s.CommodityID,
	s.Value
FROM	(
		SELECT	DefinitionDate,
			BranchID,
			CommodityID,
			ROW_NUMBER() OVER (PARTITION BY DefinitionDate, BranchID, CommodityID ORDER BY DefinitionDate) AS recID,
			Value
		FROM	tblSource
	) AS s
WHERE	s.recID = 1
	AND NOT EXISTS	(
				SELECT	*
				FROM	tblTarget AS t
				WHERE	t.DefinitionDate = s.DefinitionDate
					AND t.BranchID = s.BranchID
					AND t.CommodityID = s.CommodityID
			)

UPDATE		t
SET		t.Accum = t.Accum + s.Value
FROM		tblTarget AS t
INNER JOIN	(
			SELECT	DefinitionDate,
				BranchID,
				CommodityID,
				ROW_NUMBER() OVER (PARTITION BY DefinitionDate, BranchID, CommodityID ORDER BY DefinitionDate) AS recID,
				Value
			FROM	tblSource
		) AS s ON s.DefinitionDate = t.DefinitionDate
			AND s.BranchID = t.BranchID
			AND s.CommodityID = t.CommodityID
WHERE		s.recID > 1

DECLARE	@Target TABLE
	(
		a INT,
		b INT,
		c INT,
		i INT
	)

INSERT	@Target
SELECT	1, 1, 1, 100

DECLARE	@Source TABLE
	(
		a INT,
		b INT,
		c INT,
		i INT
	)

INSERT	@Source
SELECT	1, 1, 1, 50 UNION ALL
SELECT	3, 3, 3, 10 UNION ALL
SELECT	2, 2, 2, 45 UNION ALL
SELECT	2, 2, 2, 40

-- Peso
MERGE	@Target AS t
USING	(
		SELECT		a,
				b,
				c,
				SUM(i) AS i
		FROM		@Source
		GROUP BY	a,
				b,
				c
	) AS s ON s.a = t.a
		AND s.b = t.b
		AND s.c = t.c
WHEN	MATCHED THEN UPDATE SET t.i += s.i 
WHEN	NOT MATCHED THEN INSERT (a, b, c, i) VALUES (a, b, c, i);

SELECT	*
FROM	@Target

DECLARE	@Target TABLE
	(
		a INT,
		b INT,
		c INT,
		i INT
	)

INSERT	@Target
SELECT	1, 1, 1, 100

DECLARE	@Source TABLE
	(
		a INT,
		b INT,
		c INT,
		i INT
	)

INSERT	@Source
SELECT	1, 1, 1, 50 UNION ALL
SELECT	3, 3, 3, 10 UNION ALL
SELECT	2, 2, 2, 45 UNION ALL
SELECT	2, 2, 2, 40

-- Peso
UPDATE		t
SET		t.i = t.i + s.i
FROM		@Target AS t
INNER JOIN	(
			SELECT		a,
					b,
					c,
					SUM(i) AS i
			FROM		@Source
			GROUP BY	a,
					b,
					c
		) AS s ON s.a = t.a
			AND s.b = t.b
			AND s.c = t.c

INSERT		@Target
		(
			a,
			b,
			c,
			i
		)
SELECT		a,
		b,
		c,
		i
FROM		(
			SELECT		a,
					b,
					c,
					SUM(i) AS i
			FROM		@Source
			GROUP BY	a,
					b,
					c
		) AS s
WHERE		NOT EXISTS (SELECT * FROM @Target AS x WHERE x.a = s.a AND x.b = s.b AND x.c = s.c)

SELECT	*
FROM	@Target