>Update Table_Inv Set QTY = QTY + TABLE_DETAILS.Qty from Table_Inv INNER JOIN Table_Details ON Table_Inv.Code = Table_Details.Code


Try it :-)

DECLARE @Test  TABLE (Id int, Qty Int)
DECLARE @Test1 TABLE (Id int, Qty Int)



INSERT INTO @Test VALUES (1, 10)

INSERT INTO @Test1 VALUES (1, 50)
INSERT INTO @Test1 VALUES (1, 60)


UPDATE Test SET Test.Qty = Test.Qty + Test1.Qty
FROM @Test Test
INNER JOIN @Test1 Test1 ON Test.Id = Test1.Id


SELECT * FROM @Test ---- Qty = 70 HA!!!!