Hi,
You have 2 different ways to achieve your goal.
1)
<pre lang="SQL">SELECT ((SELECT Sum(SaleAmount) FROM Sale) - (SELECT Sum(purchaseamount) FROM Purchase))
2) Above query will not look nice to read. So you can separate both the queries in variables and use them
Declare @SalesAmount money = (SELECT Sum(SaleAmount) FROM Sale)
Declare @PurchaseAmount money = (SELECT Sum(purchaseamount) FROM Purchase)
SELECT @SalesAmount - @PurchaseAmount
Please let me know if yo have any concern or query or if you need any other help.
Thanks
Advay Pandya