Financial report query

I have been tasked with creating financial reports. I am looking for any suggestions on improving performance, readability/formatting or best practices.

```
/setting dummy parameters /
SET DATEFORMAT dmy
DECLARE @startProduct NVARCHAR = ''
DECLARE @endProduct NVARCHAR = 'zzzzzzzzzzzzzzzzzzzzzz'
DECLARE @startGroup NVARCHAR = ''
DECLARE @endGroup NVARCHAR = 'zzzzzzzzzzzzzzzzzz'
DECLARE @startDepartment NVARCHAR = ''
DECLARE @endDepartment NVARCHAR = 'zzzzzzzzzzzzzzzzzzzzzzz'
DECLARE @startDate DATE = '01/05/2000'
DECLARE @endDate DATE = CAST('31/05/2012' AS DATE)
DECLARE @StartTime TIME = '08:00:00'
DECLARE @endTime TIME = '17:00:00'
DECLARE @grouping INT = 0
DECLARE @site INT = 21
/*
ProductCode = 0
ProductGroup = 1
Department = 2
Pump = 3
Date = 4
*/

SELECT
CASE @grouping
WHEN 0 THEN P.Code
WHEN 1 THEN P.[Group]
WHEN 2 THEN P.Department
WHEN 3 THEN P.[Description]
WHEN 4 THEN CONVERT(VARCHAR,RPS.SalesDateTime,103)
END AS GroupCode,
CASE @grouping
WHEN 0 THEN P.[Description]
WHEN 1 THEN PG.[Description]
WHEN 2 THEN PD.[Description]
WHEN 3 THEN 'PUMP: ' + CONVERT(VARCHAR,RPS.PumpId) +' HOSE: ' + CONVERT(VARCHAR,RPS.HoseId)
WHEN 4 THEN CONVERT(VARCHAR(MAX),CAST(FLOOR(CAST(RPS.SalesDateTime AS FLOAT)) AS DATETIME))
END AS [GroupDescription],
SUM(RPS.Quantity) AS Quantity,
SUM( CASE
WHEN P.GSTable = 1 THEN RPS.TotalAmount / 1.1
ELSE RPS.TotalAmount
END) AS AmountX,
SUM(CASE
WHEN P.GSTable = 1 THEN (rps.TotalAmount * .10)
ELSE 0 END
) AS GST,
SUM(RPS.UnitLastCost * RPS.Quantity) AS CostXgst,
SUM(RPS.TotalAmount) AS AmountInc,
FLOOR(ROUND(((1 -
(
SUM(RPS.UnitLastCost * RPS.Quantity)
/
NULLIF(SUM
(
CASE
WHEN P.GSTable = 1 THEN RPS.TotalAmount / 1.1
ELSE RPS.TotalAmount

It appears that you're using the same expressions for the GroupCode and GroupDescription columns in the GROUP BY clause. That's asking for trouble later on if someone needs to modify either expression, especially given their size. Instead, use a subquery:

SELECT
    GroupCode,
    GroupDescription,
    -- etc
FROM
    (
        SELECT
            CASE /* etc */ END AS GroupCode,
            CASE /* etc */ END AS GroupDescription,
            -- etc
        FROM
            -- etc
        WHERE
            -- etc
    ) AS dummy
GROUP BY
    GroupCode,
    GroupDescription
ORDER BY
    GroupCode,
    GroupDescription

EDIT:

There may be some other opportunities to use subqueries to reduce duplication of expressions. For example, the expression for the column AmountX is also used in the Margin calculation.

StackExchange Code Review Q#12345, answer score: 2