Logins are used to identify (authenticate) you to the SQL
Server instance. Users are defined at the database level and are linked by means of a Security Identifier
(SID) that??™s recorded when the login is created. If Windows Authentication is used, this SID is the actual
SID that??™s used on the operating system. If SQL Authentication is used, the SID is generated randomly,
based in part on the internal ID of the SQL Server instance.
If you move a database from one server to another, all of the users will appear to have ???disappeared???
from the GUI. If you try to add a user, you??™ll get a message saying that user already exists. If you
drop and re-add the user, you??™ll lose all of the object-level permissions. If you use Windows Authentication
and the server is in the same domain, you can simply re-add the Windows user or group as a login.
The user will reappear with all of his object-level permissions intact, because the SID generated when
the Windows user or group was added as a login is exactly the same as it was on the previous server.
If you happen to use SQL Authentication, you aren??™t so lucky.
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