Question

I have a query that takes too long to execute.
I need to know if there is any optimum way of executing that to reduce execution time.

my query is

SELECT     TOP (2) ID,
                   (
                    SELECT     SUM(CurrentStock) AS SimilarItemQuantity
                    FROM          Inventory AS T1
                    WHERE      (Inventory.ProductName = ProductName)
                   ) 
                   AS Expr1
FROM         Inventory

Consider that for 20 records and it takes 15 seconds.
Is there a more faster way of doing it.

Answer 1

Correlated subquerties are a SQl antipattern, they can almost always be replaced by joins and speed up the process. You should add an order by clause anytime you are selecting top X or the results will not be consistent.

SELECT TOP 2 I1.ID, SUM(I2.CurrentStock) AS SimilarItemQuantity  
FROM   Inventory I1
join   Inventory I2 on I1.ProductName = I2.ProductName
GROUP BY I1.ID
ORDER BY I1.ID