Update 2017-06-20
This does not work on Sql Azure. I believe the replacement is
alter user
Original Article
Do you ever move a database from one server to another? If you do chances are your user ids will get out of sync since the access is granted to a sid instead of a username. If after restoring your database you cannot login you can run the following command to see a list of orphaned user ids in your database.
sp_change_users_login ‘report’
Once you see the name of the orphaned user you can choose a user on the new server to connect it to. Run the following script to link the old user to the new user.
sp_change_users_login ‘update_one’,’OriginalUserFromFirstServer’,’DestinationUserOnNewServer’
I normally like these user names to match so the second and third arguments are almost always the same.