Restore a Database into a Current Database and merge

I want to know how, or if it is possible, to backup a database then should the user wish, restore that database at a later date and merge it with the one they are currently using. Both Databases will be using the same tables and have the same fields, the difference is in the data. Ideally I would like to do this through SQL and not have to get a third party application. Any ideas or suggestions, I'm kind of in a bind here.

If you have Visual Studio Ultimate or Premium, you can use "Data Compare" feature in Data menu. It can compare data between two tables and have ability to directly update your database or generate update script.

StackExchange Database Administrators Q#19210, answer score: 2