Optimization: Subqueries And Backreferences?

I wonder if somebody could give me advice on SQL Server7-specific handling of subqueries and backreferences...

What is a) more efficient, and b) faster:

1. IF (DATEDIFF(minute, (
SELECT tLastHitTime
FROM (
SELECT *
FROM tblSession
WHERE tSessionID = 'abcd1234'
) as sess), getDate()) < 30)
SELECT *
FROM sess

or:

2. IF (DATEDIFF(minute, (
SELECT tLastHitTime
FROM tblSession
WHERE tSessionID = 'abcd1234'
) as sess), getDate()) < 30)
SELECT *
FROM tblSession
WHERE tSessionID = 'abcd1234'


In the first example, 2 SELECT statements (line 2 & line 8) refer to derived table by its alias 'sess'. In the 2nd example, both queries are independant, yet identical...

I wonder if SQLServer is "smart" enough to execute identical queries only once, and then reuse the resulset (e.g. derived table), or it requires explicit references (such as alias) in order to do so...

TIA,
Alex