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Understanding and dealing with orphaned users in a SQL Server database
Attaching and restoring databases from one server instance to another are common tasks executed by a DBA. After attaching or restoring of a database, previously created and configured logins in that database do not provide access. The most common symptoms of this problem are that the application may face login failed errors or you may get a message like the user already exists in the current database when you try to add the login to the database. This is a common scenario when performing an attach or a restore, so how do you resolve this?