https://www.simple-talk.com/sql/learn-sql-server/the-merge-statement-in-sql-server-2008/

http://msdn.microsoft.com/en-us/library/bb510625.aspx

read the best explanation by Pinal : http://blog.sqlauthority.com/2008/08/28/sql-server-2008-introduction-to-merge-statement-one-statement-for-insert-update-delete/