how to create an sql query to getting profit of each product

You can try something like this. (not tested)

SELECT 
A.ProductID, 
A.PurchasePrice, 
B.SalesPrice, 
CAST((B.SalesPrice - A.PurchasePrice) AS DECIMAL(10,2))/CAST(A.PurchasePrice AS DECIMAL(10,2))  AS [ProfitValue %age] 
FROM tblPurchase A 
INNER JOIN tblSales B ON A.ProductID = B.ProductID

In case if you have multiple ProductID you can try aggregating using GROUP BY clause.

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Regards, RSingh