The following Microsoft SQL Server T-SQL sample script demonstrates the application of derived table (cnum) to build complex queries:

-- SQL derived table - subselect - inner join subselect

USE Northwind;

SELECT   c.CategoryName,

         p.ProductName,

         p.UnitPrice,

         cnum.NoOfProducts

FROM     Categories c

         INNER JOIN Products p

           ON c.CategoryID = p.CategoryID

         INNER JOIN (SELECT   c.CategoryID,

                              NoOfProducts = count(* )

                     FROM     Categories c

                              INNER JOIN Products p

                                ON c.CategoryID = p.CategoryID

                     GROUP BY c.CategoryID) cnum  -- derived table - subselect

           ON c.CategoryID = cnum.CategoryID

ORDER BY c.CategoryName

GO

/* Partial results

CategoryName      ProductName             UnitPrice   NoOfProducts

Beverages         Chai                    18.00       12

Beverages         Chang                   19.00       12

Beverages         Guaraná Fantástica      4.50        12

Beverages         Sasquatch Ale           14.00       12

Beverages         Steeleye Stout          18.00       12

Beverages         Côte de Blaye           263.50      12

*/

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Related articles:

Using Derived Tables to Simplify the SQL Server Query Process

SQL Server 2005 Derived Tables, Common Table Expressions and Indexed Views Query Processing Part II