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  SQL Server Tips by Burleson

Derived table

A derived table is a select statement inside parenthesis, with an alias, used as a table in a join or union. Derived tables are very common with JOIN clauses because they have a defined name, which is necessary for the join, unlike subqueries. They are an alternative to temporary tables in the same situations as subqueries. Another use for derived tables is in row calculations, particularly when there are excessive aggregate functions and CASE statements.

Example: same as the previous one.

SELECT TOP 100 PERCENT dbo.titles.title, dbo.titles.royalty
FROM dbo.titles
(SELECT (min(royalty)+max(royalty))/2 as avgTotal FROM dbo.roysched ) r2
on dbo.titles.royalty> avgTotal
ORDER BY dbo.titles.title

This is an example where the JOIN clause allows you to remove the WHERE clause to filter rows. Usually it is better to use a JOIN to filter rather than a WHERE statement but, in this case, both queries are at the same performance level.

The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter  

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