Calculating Cumulative Commission Amounts - sql-server-2008

Question

I have a commission banding table containing the following:
Lower Upper Percentage
0 300 45
300.01 800 50
800.01 1500 55
The lower and upper amounts are currency values, and i need to calculate a cumulative amount to pay based on the total sales, using the percentage amount relevant to the total sales.
If i then have a total sales amount of 350, my commission should be calculated as the following:
first 300 of the total would be using the 45%
the remaining 50 would be using the 50%
so my total would be
300*45% = 135
50*50% = 25
Total = 160
I am updating a table with the amounts via a sproc so would need to accommodate this in there.
How is the best way to go about this?
Note : the sproc below has the correct column names, where as the example above i changed the names of the columns for simplicity. The SPROC also joins the table where the bands are stored, and the update table is a kind of working/reporting table
EDIT : sproc update section:
UPDATE CommissionCalculationDetails
SET TotalCommissionAmount =
case
when TotalSales > Upper then Upper
when TotalSale > Lower then #sales - Lower
else 0
end
* Percentage / 100
FROM CommissionCalculationDetails
LEFT JOIN CommissionBand
ON TotalSales > CommissionBand.Lower
AND TotalSales < CommisionBand.Upper

Answer 1

I propose that you store non-inclusive lower bounds instead (ex: 300 instead of 300.01) and use strictly greater than when comparing against it. As it stands, the value 300.005 would not be correctly categorized.
If this was the case, you could compute the total commission using the following query:
select
sum (
case
when #sales > Upper then Upper
when #sales > Lower then #sales - Lower
else 0
end
* Percentage / 100
) as TotalCommission
from CommissionTable
Here's an online test version of this: http://www.sqlfiddle.com/#!3/87f12/8
Slightly offtopic: Your table currently contains redundant information; each lower bound is (more or less) equal to the previous upper bound. Although this is not essential, you could think of a way to store, for example, upper bounds only (and have a null signifying unbounded).
For the update, one possible solution is to extract the commission calculation in a function, like so:
create function ufn_CalculateCommission(#Sales money)
returns money
as
begin
declare #result money
select
#result =
sum (
case
when #sales > Upper then Upper
when #sales > Lower then #sales - Lower
else 0
end
* Percentage / 100
)
from CommissionBand
return #result
end
After declaring the function, the update can be simplified to:
update CommissionCalculationDetails
set TotalCommissionAmount = dbo.ufn_CalculateCommission(TotalSales);
Here's how it works: http://www.sqlfiddle.com/#!3/f4405/4