One my favorite commands when I was started my career was Sp_changeuserslogin. I had to move a lot of databases around and I orphaned a lot of users. This command really helped me find what users were orphaned and reconnect them with the appropriate database.
When should you use this command?
This command is very useful when you have a user in your database and a login on your server that should be connected (or mapped) but are not.
To find out if you have an oprhaned users run:
EXEC Sp_change_users_login ‘report’
This will give you a list of orphaned users in your DB.
If you find you have some orphaned users you can run this command to fix them:
It’s a handy tool to use but BOL warns avoiding it because it will be going away soon. They suggest using ALTER USER instead.
ALTER USER YourUser WITH LOGIN = ‘YourUser’