Every fifth order entitles your customer to a discount. The amount of the discount is based upon the total amount of the four orders leading up to the fifth order.

example_get_every_5_row.sql

-- Listing 12-16: Creating a Temporary Table of Order Totals for a Customer
SELECT IDENTITY(INT, 1, 1)              AS Sequence,
       O.OrderID,
       SUM(OD.Quantity * OD.UnitPrice)  AS Total
INTO   #Totals
FROM   Orders AS O
       JOIN [Order Details] AS OD
            ON  OD.Orderid = o.OrderID
WHERE  O.CustomerID = 'SAVEA'
GROUP BY
       O.OrderID
ORDER BY
       O.OrderID

CREATE UNIQUE CLUSTERED INDEX #Idx ON #Totals(Sequence)

-- Listing 12-17: Calculating the Discount
SELECT OrderID,
       Total,
       Previous4,
       CASE 
            WHEN Previous4 < 10000.00 THEN 5
            WHEN Previous4 BETWEEN 10000.00 AND 15000.00 THEN 10
            ELSE 20
       END  AS Discount
FROM   (
           SELECT T1.OrderID,
                  T1.Total, T1.Sequence,
                  (
                      SELECT SUM(T2.Total) AS Total
                      FROM   #Totals AS T2
                      WHERE  T2.Sequence >= T1.Sequence - 4
                             AND T2.Sequence < T1.Sequence
                  )        AS Previous4
           FROM   #Totals  AS T1
           WHERE  0 = T1.Sequence % 5
       )    AS X