Reporting Previous Records on SQL Server - sql-server

Question

I’m struggling a bit here. The data is fabricated, but the query concept is very real.
I need to select the Customer, Current Amount, Previous Amount, Sequence and Date
WHERE DATE < 1190105
AND the DATE/SEQ is the maximum date/seq prior to that date point grouping by customer.
I’ve spent quite a few days trying all sorts of things using HAVING, nested select to try and obtain the max-date/amount and min-date/amount by customer and can’t quite get my head around it. I’m sure it should be quite easy, but any help you can offer would be really appreciated.
Thanks
**SEQ DATE CUSTOMER AMOUNT**
1 1181225 Bob 400
2 1181226 Fred 300
3 1190101 Bob 100
4 1190104 Fred 500
5 1190104 George 200
6 1190105 Bob 150
7 1190106 Bob 200
8 1190110 Fred 160
9 1190110 Bob 300
10 1190112 Fred 400

Answer 1

Opt 1 use row number and lag functions
SELECT
ROW_NUMBER() OVER (Partition By CustomerID Order By [Date]) as Sec,
[Date],
Customer,
Amount as CurrentAmount,
Lead(Amount) OVER (Partition By CustomerID, Order By [Date]) as PreviousAmount
FROM
YourTable
WHERE
[DATE] < 1190105
Opt use outer apply
SELECT
ROW_NUMBER() OVER (Partition By Customer Order By [Date]) as Sec,
[Date],
Customer,
Amount as CurrentAmount,
Prev.Amount as PreviousAmount
FROM
YourTable T
OUTER APPLY (
SELECT TOP 1 Amount FROM YourTable
WHERE Customer = T.Customer AND [Date] < T.[Date]
ORDER BY [DATE] DESC
) Prev
WHERE
DATE < 1190105
Opt 3 use a correlated subquery
SELECT
ROW_NUMBER() OVER (Partition By Customer Order By [Date]) as Sec,
[Date],
Customer,
Amount as CurrentAmount,
(
SELECT TOP 1 Amount FROM YourTable
WHERE Customer = T.Customer AND [Date] < T.[Date]
ORDER BY [DATE] DESC
) as PreviousAmount
FROM YourTable
WHERE
DATE < 1190105

Answer 2

First restrict the rows with the date filter, then search for the max by customer.
Using GROUP BY:
DECLARE #FilterDate INT = 1190105
;WITH MaxDateByCustomer AS
(
SELECT
T.CUSTOMER,
MaxSEQ = MAX(T.SEQ)
FROM
YourTable AS T
WHERE
T.Date < #FilterDate
GROUP BY
T.CUSTOMER
)
SELECT
T.*
FROM
YourTable AS T
INNER JOIN MaxDateByCustomer AS M ON
T.CUSTOMER = M.CUSTOMER AND
T.SEQ = M.MaxSEQ
Using ROW_NUMBER window function:
DECLARE #FilterDate INT = 1190105
;WITH DateRankingByCustomer AS
(
SELECT
T.*,
DateRanking = ROW_NUMBER() OVER (PARTITION BY T.CUSTOMER ORDER BY T.SEQ DESC)
FROM
YourTable AS T
WHERE
T.Date < #FilterDate
)
SELECT
D.*
FROM
DateRankingByCustomer AS D
WHERE
D.DateRanking = 1