--**************************************
--     
-- Name: More Efficient Queries using Vi
--     tual Tables!!
-- Description:This is a very usefull te
--     chnique which can reduce your query time
--     s exponentially on SQL Server databases.
--      A Virtual Table is a Select Query refer
--     enced like a table within the FROM claus
--     e of your select statement. By using a V
--     irtual Table, you will be able to have a
--      much more efficient query than if you w
--     ere to use a Temp Table or an IN (SELECT
--     ...) statement in your WHERE clause. Try
--      testing this out on large data sets, an
--     d you will see a significat performance 
--     gains.<BR><BR>
I know these are simple examples that can easily be replaced WITH INNER joins, but as you run INTO querying complex TABLE relationships this will be a handy tool.
-- By: David Drake
--
--This code is copyrighted and has-- limited warranties.Please see http://
--     www.Planet-Source-Code.com/xq/ASP/txtCod
--     eId.283/lngWId.5/qx/vb/scripts/ShowCode.
--     htm--for details.--**************************************
--     

EXAMPLE 1 (Not using a Virtual Table):
 SELECT *
 FROM TableA
 WHERE ID IN (SELECT ID FROM TableB)
EXAMPLE 2 (Using a Virtual Table):
 SELECT A.*
 FROM TableA A,
(SELECT ID
FROM TableB) AS B
WHERE A.ID = B.ID
EXAMPLE 3 (Nesting Virtual Table):
 SELECT A.*
 FROM TableA A,
(SELECT ID
FROM TableB T,
(SELECT cID
FROM TableC) AS C
WHERE T.ID = C.cID) AS B
 WHERE A.ID = B.ID