SQL DISTINCT Column with 2nd Criteria as datetime - sql

Question

I have on just started learning SQL in SQL Server Management Studio and getting thrown into the deep end.
I just need unique DriverID that has a LogoffTime in the last 3 month, with the headings included below.
What I have so far:
SELECT
Dr.DriverName, Dr.DriverNumber, Dr.DriverID,
DL.DriverID, DL.LogoffTime,
ROW_NUMBER() OVER (PARTITION BY DL.DriverID ORDER BY DL.LogoffTime DESC) AS rn
FROM
Taxihistory.dbo.DriverLogon DL, Taxihistory.dbo.Driver Dr
WHERE
DL.DriverID = Dr.DriverID
AND DL.LogoffTime <= '20180931'
AND rn = 1
ORDER BY
DL.LogoffTime DESC;
I am currently getting this error:
Msg 207, Level 16, State 1, Line 7
Invalid column name 'rn'

Answer 1

In case you want to explore CTE (Common Table Expression) option, you may also be able to achieve this with CTE. You can try something like below:
WITH CTE AS (
SELECT dr.drivername,
dr.drivernumber,
dr.driverid,
dl.logofftime,
row_number() OVER (PARTITION BY dl.driverid
ORDER BY dl.logofftime DESC) AS rn
FROM taxihistory.dbo.driverlogon dl
INNER JOIN taxihistory.dbo.driver dr
ON dr.driverid = dl.driverid
WHERE dl.logofftime <= Convert(datetime, '2018-09-30') )
SELECT tbl.drivername,
tbl.drivernumber,
tbl.driverid,
tbl.logofftime
FROM CTE tbl
WHERE tbl.rn = 1
ORDER BY tbl.logofftime DESC;

Answer 2

You cannot use column aliases in the WHERE clause. Neither can you use row_number() there. You have to wrap the query with the row_number() in a subquery and select from that.
SELECT x.drivername,
x.drivernumber,
x.driverid,
x.logofftime
FROM (SELECT dr.drivername,
dr.drivernumber,
dr.driverid,
dl.logofftime,
row_number() OVER (PARTITION BY dl.driverid
ORDER BY dl.logofftime DESC) rn
FROM taxihistory.dbo.driverlogon dl
INNER JOIN taxihistory.dbo.driver dr
ON dr.driverid = dl.driverid
WHERE dl.logofftime <= '20180930') x
WHERE x.rn = 1
ORDER BY x.logofftime DESC;
It is also advisable to use explicit join syntax. And I do hope, that driverlogon.logofftime is not an [n][var]char but some date/time type.

Answer 3

You should get the logon from the last 3 months up to the current date by doing so:
SELECT Dr.DriverName, Dr.DriverNumber, Dr.DriverID, DL.DriverID, DL.LogoffTime
FROM Taxihistory.dbo.DriverLogon DL,
INNER JOIN Taxihistory.dbo.Driver Dr ON DL.DriverID = Dr.DriverID
WHERE DL.LogOffTime < DATEADD(MONTH, -3, GETDATE())
ORDER BY DL.LogoffTime DESC;