I was talking to a buddy this morning and he'd never seen the syntax for a derived table in MSSQL before. He couldn't quite grasp why they might be useful and I was having difficulty explaining via instant messenger, so I thought I'd blog a real world example of how derived tables can be useful.
In very basic terms, a derived table is a virtual table that's calculated on the fly from a select statement. They can be tremendously useful in certain situations. For my example below, I'm going to use the Northwind database that included with MSSQL 2000.
Let's say you've been asked to generate a report that shows off the total number of orders each customer placed in 1996. "Not a problem." you think to yourself. "This is just an easy aggregated join query." So, you sit down and generate your query and come up with: