SQL cache invalidation enables you to make the cache entry 
dependent on the database, so the cache entry will only be 
cleared when data in the database is changed.

Polling-based Invalidation
This mechanism uses polling to check if a table has been 
updated since the page was cached.

SQL Server 2005 Notification-based Cache Invalidation
This mechanism uses the query change notification mechanism 
of Sql Server 2005 to detect changes to the results of 
queries. Unlike polling based invalidation for Sql Server 
7.0 and 2000, notification based invalidation requires much 
less setup. 

1. Unlike polling based validation, no <sqlCacheDependency> 
needs to be registered in your application's configuration. 
Furthermore, no special configuration using the 
aspnet_regsql.exe tool is needed. 

2. A notification based dependency is configured on the 
OutputCache directive using the string CommandNotification. 
This value indicates to ASP.NET that a notification based 
dependency should be created for the page or datasource 
control.

3. System.Data.SqlClient.SqlDependency.Start() method must 
be called somewhere in the application before the first SQL 
query is executed. This method could be placed in 
Application_Start() event in global.asax file. 

Whenever a command is issued to Sql Server 2005, ASP.NET 
and ADO.NET will automatically create a cache dependency 
that listens to change notifications sent from the SQL 
Server. As data is changed in Sql Server, these 
notifications will cause the cached queries to be 
invalidated on the web server. The next time a page or 
datasource control associated with the dependency is 
requested, the page or datasource control will be executed 
again as opposed to serving cached information.