Read this SQL tutorial to learn when to use SELECT, JOIN, subselects and UNION to access multiple tables with a single statement.

Ok, I'm trying to write a query in a stored procedure in SQL Server 2000. Because some of the parts of the query vary based on certain situations I construct a string that contains the query and ...

An alternative - again, if supported - would be to create a view that works the same as the derived table, and use this in your query instead.