How to update the single row twice. Second update should take the updated value from 1st update

Question

I have two tables tblProducts and temptable. temptable contains the details of sold products stock quantity. I want to update sold quantity of products in tblproduct. My problem is temp table will have multiple records for same product and when I update it using join it update on the old value instead of new value.

tblProduct         tempTable
==============     ==========
ProductId            Id
Name                 ProductId
StockQuantity        SoldQuantity
                     OrderType

--------------
tblProduct
--------------
ProductId     Name         TotalSoldQuantity
1          Product1           10    
2          Product2           20

-------------
tempTable
-------------
Id      ProductId   SoldQuantity  OrderType
1          1             5           1
2          1             5           2

I have written below query :

 UPDATE P SET P.TotalSoldQuantity = P.TotalSoldQuantity + T.SoldQuantity 
 FROM tempTable T JOIN tblProduct P ON P.ProductId = T.ProductId

This query update the product1's TotalSoldQuantity to 15 instead of 20. It should update one by one after taking the previous records updated value.

Please help.

Answer 1

You need to aggregate SoldQuantity first to get total sold and then update:

WITH TotalAgg AS
(SELECT T.ProductId, SUM(T.SoldQuantity) AS TSQA FROM tempTable AS T GROUP BY T.ProductId)
UPDATE P SET P.TotalSoldQuantity = P.TotalSoldQuantity + T.TSQA 
FROM TotalAgg T JOIN tblProduct P ON P.ProductId = T.ProductId

Answer 2

I would write it like this, avoiding the non-standard UPDATE ... FROM syntax, and providing a simple pattern for examining the changes before applying them:

with q as
(
  select p.ProductId, 
         p.TotalSoldQuantity, 
         AdditionalSoldQuantity = (select sum(SoldQuantity) from tempTable where productId = p.ProductId) 
  from tblProduct p
)
--select * from q
update q set TotalSoldQuantity = TotalSoldQuantity + AdditionalSoldQuantity 

Answer 3

You need to aggregate your quantitys per ProductId in your temp table and join that to Products.

 UPDATE P 
 SET P.TotalSoldQuantity = P.TotalSoldQuantity + T.SoldQuantity 
 FROM 
    (SELECT ProductId, SUM(TotalSoldQuantity) FROM tempTable GROUP BY ProductId) T JOIN 
    tblProduct P ON P.ProductId = T.ProductId