Update table from another table in SQL is giving an unexpected result - sql

Question

Table Items :
ItemCode---Name-------Stock
111-----------Book---------10
112-----------Bag-----------03
113-----------Pen-----------08
114-----------Pencil--------13
Table INVOICEITEMS(SoldItems) :
BillNumber---ItemCode-----UnitValue(Qty)
1005-----------111------------------3
1005-----------111------------------2
1005-----------113------------------4
1006-----------111------------------6
1007-----------112------------------5
We need to update the ITEMS table from INVOICEITEMS table.
We need to add to the Stock of each ItemCode in ITEMS table from the sum of the UnitValues of BillNumber '1005' in INVOICEITEMS.
Query used :
UPDATE ITEMS SET Stock=Stock+(SELECT SUM(T.UnitValue) FROM INVOICEITEMS T WHERE T.BillNumber LIKE '1005' AND T.ItemCode LIKE ItemCode) WHERE ItemCode IN(SELECT J.ItemCode FROM INVOICEITEMS J WHERE J.BillNumber LIKE '1005')
Expected Result After updation :
Table ITEMS
ItemCode---Name----Stock
111-----------Book-------15
112-----------Bag---------03
113-----------Pen---------12
114-----------Pencil------13
Result we are getting:
Table ITEMS
ItemCode---Name----Stock
111-----------Book--------19
112-----------Bag---------03
113-----------Pen---------17
114-----------Pencil-------13
Please help in correcting this query.

Answer 1

One way to investigate problems with update statements is to convert them to select statements. For your update statement, I come up with the following query:
SELECT Items.*
,(SELECT SUM(T.UnitValue) FROM INVOICEITEMS T WHERE T.BillNumber LIKE '1005' AND T.ItemCode LIKE ItemCode)
FROM Items
WHERE ItemCode IN(SELECT J.ItemCode FROM INVOICEITEMS J WHERE J.BillNumber LIKE '1005')
As you can see, the last column is 9 for every row, which isn't surprising because you have not done anything to return different values based on which row you're looking at.
You can then change the select statement to return the data you do want:
SELECT Items.ItemCode
,Items.Stock
,SUM(T.UnitValue)
,Items.Stock + SUM(T.UnitValue)
FROM Items
JOIN INVOICEITEMS T ON T.ItemCode = Items.ItemCode
WHERE T.BillNumber = 1005
GROUP BY Items.ItemCode, Items.Stock
And finally you can incorporate that back into an UPDATE statement:
UPDATE Items SET Stock = NewSum
FROM Items JOIN
(SELECT Items.ItemCode
,Items.Stock
,Items.Stock + SUM(INVOICEITEMS.UnitValue) NewSum
FROM Items
JOIN INVOICEITEMS ON INVOICEITEMS.ItemCode = Items.ItemCode
WHERE INVOICEITEMS.BillNumber = 1005
GROUP BY Items.ItemCode, Items.Stock
) T ON T.ItemCode = Items.ItemCode
It might then be prudent to extract hard-coded values from the sub-query and make them part of the higher-level query. One reason this might be a good idea is because then the sub-query is more portable. Then you'd end up with this:
UPDATE Items SET Stock = NewSum
FROM Items JOIN
(SELECT Items.ItemCode
,INVOICEITEMS.BillNumber
,Items.Stock
,SUM(INVOICEITEMS.UnitValue) InvoiceQty
,Items.Stock + SUM(INVOICEITEMS.UnitValue) NewSum
FROM Items
JOIN INVOICEITEMS ON INVOICEITEMS.ItemCode = Items.ItemCode
GROUP BY Items.ItemCode, Items.Stock, INVOICEITEMS.BillNumber
) T ON T.ItemCode = Items.ItemCode
WHERE T.BillNumber = 1005

Answer 2

AND T.ItemCode LIKE ItemCode line in the SQL is not getting executed as ItemCode hasn't been specified in the SQL before the execution of the inner SQL statement.
Associate ItemCode explicitly with the ITEMS table and it would work.

Answer 3

Well, in this case you could use a Stored Procedure or a Function:
e.g. Define a Function
CREATE FUNCTION getItemSum(#BillID int, #ItemID int)
RETURNS int
AS
-- Returns the stock level for the product.
BEGIN
DECLARE #ret int;
SELECT #ret = SUM(UnitValue)
FROM INVOICEITEM p
WHERE p.ITEMCODE = #ItemID
AND p.BillNumber = #BillID;
IF (#ret IS NULL)
SET #ret = 0;
RETURN #ret;
END;
GO
And Call SQL Statement like this:
UPDATE ITEMS
SET Stock=Stock + getItemSum(1005, ItemCode)
GO

Answer 4

One way to do this is to use a join with a sub-query:
UPDATE items
SET Stock += item_qty_sum
FROM items i
INNER JOIN (
SELECT itemcode, SUM(unitvalue) AS item_qty_sum
FROM INVOICEITEMS
WHERE BillNumber = 1005
GROUP BY ItemCode
) ii
ON ii.ItemCode = i.ItemCode
Sample SQL Fiddle showing the query in action giving the desired results.