**1) The following SQL statement is working**
SELECT

SUM(sales.TotalAmount) AS the_sum_Total

FROM

sales

WHERE

LEFT(sales.InvDate ,6) = {paramMonth}

**2) But the moment i need to deduct the discount amount from Total Amount, the_sum_total will return NULL**
SELECT

SUM((sales.TotalAmount - sales.DiscountAmount)) AS the_sum_Total

FROM

sales

WHERE

LEFT(sales.InvDate ,6) = {paramMonth}

Is there a bug in the SQL query where i cannot deduct within the SUM?

Regards,

Jon Lau