After few posts devoted to indexes it’s time to look at something else. Let’s see some common problems with users and logins, and methods of overcoming them. These are the basic and most frequently occuring ones.

1. Unable to connect to server - cannot open user default database (error 4064).
   
   
   The message appears if the default database for a login does not exist or it’s unavailable. An example of former would be a migration, when you move some databases and logins between servers. Latter is a little bit more tricky – imagine a server which participates in database mirroring; the mirror database is in recovery all the time, so any attempts of logging into it would be rejected.
   The solution is easy – set the default database for the login to an existing one, which always available. Good practice is to use master for this purpose – master always exists and all users can connect to it. You can use SSMS for it or – if you prefer typing:

   ALTER LOGIN [testLogin] WITH DEFAULT_DATABASE=[master]

2. Unable to map login to user in a database - user, group or role already exists in the current database (error 15023).
   
   
   This message can also appear during a migration, if you copy (or move) your database (using Copy Database Wizard or detach/attach) but forget to create respective logins first, rather create logins after the database is brought online, and then try to map them to database users.
   The solution is also simple – but only T-SQL this time. You have to execute it in the database context and it works only for SQL logins (see Books Online for details of sp_change_users_login):

   sp_change_users_login 'update_one', login_name, user_name

   
   There is also a note in mentioned BOL article saying that from SQL Server 2008 sp_change_users_login stored procedure is considered to be removed in the future. Instead, it is recommended to use ALTER USER statement:

   ALTER USER user_name TO LOGIN = login_name

3. Orphaned database (no owner set).
   
   I can’t think of any reasonable explanation for that, but since I have experienced that recently, it’s worth mentioning. You restore a database from backup and it turns out that user dbo is mapped to a login that does not exist. You can do it from SSMS (Database Properties –> Files –> input database owner) or using T-SQL:

   sp_changedbowner login_name

   If you look for sp_changedbowner in the Books Online, you'll notice that it is marked for deprecation as well. The suggested solution is to use ALTER AUTHORIZATION insted:

   ALTER AUTHORIZATION ON DATABASE::DBOUserTest to [login_name]

A few words about required privileges to complete these actions. ALTER LOGIN requires ALTER ANY LOGIN permission, but a principal can change it for its own login (this may fail if tried from SSMS since it sets CHECK_EXPIRATION and CHECK_POLICY which require CONTROL SERVER permission, at least it’s what I observed on SQL Server 2005). To remap a user with ALTER USER, you require CONTROL permission in a database. sp_change_users_login requires membership in db_owner database role. Finally, ALTER AUTHORIZATION requires TAKE OWNERSHIP on the database (on the object, in general) and might require IMPERSONATE, if you’re not logged with login taking ownership. sp_changedbowner might need CONTROL SERVER in addition to that, if new owner doesn’t have a user in a database. Since a DBA probably is a sysadmin, this is not a problem, but I have seen users in live databases yielding CONTROL which might make us wonder “do they really need it?”.

Well, this would be all about users and logins for now. If you have questions, suggestions or corrections, feel free to comment and ask. I’ll be happy to respond.