TSQL Union, Max(date) - have solution. Want improvement - sql-server

Question

The following query works, but there has to be a better way to set the value of a table to the max date of the union of two sets of data. Here's what I have:
Update stagingTable
Set OverrideFlag =
(
select total.overrideflag from
(
select Customer_SSN as ssn, RequestDateTime as maxdate, overrideflag
from tableA
where RequestDateTime > '9/1/2012'
union
select ssn, EntryDate as maxdate, overrideflag
from tableB
where EntryDate > '9/1/2012'
) total
join
(
select ssn, max(maxdate) as maxdate from
(
select Customer_SSN as ssn, RequestDateTime as maxdate
from tableA
where RequestDateTime > '9/1/2012'
union
select ssn, EntryDate as maxdate
from tableB
where EntryDate > '9/1/2012'
) maxs
group by ssn
) maxdates on total.ssn = maxdates.ssn and total.maxdate = maxdates.maxdate where total.ssn = stagingTable.ssn
)

Answer 1

It appears you are doing the exact same thing twice, so I am not getting the need to define something twice and join it back to itself unless there was something different in one of the nested selects. You are essentially writing the same statement twice and the redundancy may be an issue as one of the selects appears completely redundant.
-- this is a CTE and is better for reuse than a nested select as you can reference it
-- is as a base and reuse that, versus having to write the same statement twice.
;with a as
(
select
Customer_SSN as ssn,
RequestDateTime as maxdate,
OverRideFlag,
-- EDIT, you should be able to use a 'Windowed function' to get the maxDate
max(RequestDateTime) over(partition by SSN order by RequestDateTime desc) as maxDate
from tableA
where RequestDateTime > '9/1/2012'
union
select
ssn,
EntryDate,
OverRideFlag,
max(RequestDateTime) over(partition by SSN order by RequestDateTime desc) as maxDate
from tableB
where EntryDate > '9/1/2012'
)
Update stagingTable
Set OverrideFlag = total.overrideflag
from a total
join stagingTable on total.ssn = stagingTable.ssn
-- do not know reference here so you need to add that table as a 'join'
where total.EntryDate = total.maxDate

Answer 2

I also found a different way to do it with a temp table. I'm becoming very comfortable with these, but I always want to see a different way how to do this. Not disappointed!
create table #tOvr(customerID varchar(15), ssn varchar(11), EntryDate datetime, overrideflag varchar(2))
insert into #tOvr
select customer_ID, Customer_SSN, RequestDateTime, overrideflag
from tableA
where RequestDateTime > '9/1/2012'
and Customer_ID in
(select contact_ID from stagingTable
where Location_ID = #Location_ID)
insert into #tOvr
select Customer_ID, ssn, EntryDate, overrideflag
from tableB
where EntryDate > '9/1/2012'
and Customer_ID in
(select contact_ID from stagingTable
where Location_ID = #Location_ID)
Update stagingTable
Set OverrideFlag =
(select overrideflag from #tOvr
where EntryDate = (select max(EntryDate) from #tOvr where #tOvr.customerID = stagingTable.contact_ID)
)