More Efficient Queries using Vitual Tables!!

				
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-- Name: More Efficient Queries using Vitual Tables!!
-- Description:This is a very usefull technique which can reduce your query times exponentially on SQL Server databases. A Virtual Table is a Select Query referenced like a table within the FROM clause of your select statement. By using a Virtual Table, you will be able to have a much more efficient query than if you were to use a Temp Table or an IN (SELECT...) statement in your WHERE clause. Try testing this out on large data sets, and 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
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--This code is copyrighted and has-- limited warranties.Please see http://www.Planet-Source-Code.com/vb/scripts/ShowCode.asp?txtCodeId=283&lngWId=5--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