Just for me to remember. I'm currently doing migration of our applications and databases from the East Coast going to California. This is something that came up when I migrated one of my databases. I certainly would like to remember...
This is what happens when you restore the database to a new server, you expect that accounts could be configured to access the database via the "User Mapping". But instead there is this error message: "Error 15023: User or role already exists in the current database." Apparently, this is just a case of SID mismatch. Here's the solution:
SQL Server has a built in stored procedure to handle this issue:
Use the parameter 'Update_One' to update one SID:
USE YourDatabase GO EXEC sp_change_users_login Update_One, 'yourLogin', 'yourLogin' GO
Go to this post for further reading.