Question

I have a table containing transaction information on items. The possible transactions are purchase (type_id: 1) and sale(type_id: 2). A really stripped down version of the table “Transactions” is like the following:

Transaction_ID  Item_ID  Transaction_Type_ID  Quantity  Price  Date
1,               1,        1,                    50,        10,     6/1,
2,               2,        1,                    40,        20,     13/1,
3,               1,        2,                    10,        13,     14/1,
4,               2,        2,                    20,        25,     3/2,
5,               1,        2,                    20,        12,     20/2

I have the following query:

SELECT B.Item_ID
       B.Quantity - (SELECT SUM(Quantity) FROM Transactions A Where A.Item_ID = B.Item_ID AND A.Transaction_Type_ID = 2) AS 'Quantity Left'
       B.Price * (B.Quantity - (SELECT SUM(Quantity) FROM Transactions A Where A.Item_ID = B.Item_ID AND A.Transaction_Type_ID = 2)) AS 'Purchase Amount Left'
FROM   Transaction B
WHERE  B.Transaction_Type_ID = 1
AND    B.Quantity - (SELECT SUM(Quantity) FROM Transactions A Where A.Item_ID = B.Item_ID AND A.Transaction_Type_ID = 2) > 0

Like you may already noticed, I am trying to get all the purchased items that are still in stock. You may notice also that there is an annoying sub-query repeating twice in the SELECT clause and once in the WHERE clause.

How can I reduce it? Is it possible to use WITH in the beginning of the statement in this case?

Answer 1

JOIN onto an aggregated derived table?

SELECT 
    B.Item_ID
    B.Quantity - ISNULL(A.SUMQuantity, 0) AS 'Quantity Left',
    B.Price * (B.Quantity - ISNULL(A.SUMQuantity, 0)) AS 'Purchase Amount Left'
FROM  
    Transaction B
    LEFT JOIN
    (
     SELECT SUM(Quantity) AS SUMQuantity, Item_ID
     FROM Transaction 
     WHERE Transaction_Type_ID = 2
     GROUP BY Item_ID
    ) ON A.Item_ID = B.Item_ID
WHERE  
    B.Transaction_Type_ID = 1
    AND   
    B.Quantity - ISNULL(A.SUMQuantity, 0) > 0

If you always have rows where Transaction_Type_ID = 2, then you can remove LEFT JOIN and ISNULL. In your current code, you assume you always have rows.

It also looks like you’re mixing entities in the same table based on Transaction_Type_ID. A simple SUM(Quantity) .. GROUP BY Item_ID would be more correct if
– sales are <0 quantity transaction
– restocks are >0 quantity transaction2