[ @Action = ] 'action'

Describes the action to be performed by the procedure. action is varchar(10). action can have one of the following values.

Value	Description
Auto_Fix	Links a user entry in the sys.database_principals system catalog view in the current database to a SQL Server login of the same name. If a login with the same name does not exist, one will be created. Examine the result from the Auto_Fix statement to confirm that the correct link is in fact made. Avoid using Auto_Fix in security-sensitive situations. When you use Auto_Fix, you must specify user and password if the login does not already exist, otherwise you must specify user but password will be ignored. login must be NULL. user must be a valid user in the current database. The login cannot have another user mapped to it.
Report	Lists the users and corresponding security identifiers (SID) in the current database that are not linked to any login. user, login, and password must be NULL or not specified.
Update_One	Links the specified user in the current database to an existing SQL Server login. user and login must be specified. password must be NULL or not specified.

[ @UserNamePattern = ] 'user'

Is the name of a user in the current database. user is sysname, with a default of NULL.

[ @LoginName = ] 'login'

Is the name of a SQL Server login. login is sysname, with a default of NULL.

[ @Password = ] 'password'

Is the password assigned to a new SQL Server login that is created by specifying Auto_Fix. If a matching login already exists, the user and login are mapped and password is ignored. If a matching login does not exist, sp_change_users_login creates a new SQL Server login and assigns password as the password for the new login. password is sysname, and must not be NULL.

Security Note:
Use a strong password. For more information, see Strong Passwords.

0 (success) or 1 (failure)

Use sp_change_users_login to link a database user in the current database with a SQL Server login. If the login for a user has changed, use sp_change_users_login to link the user to the new login without losing user permissions. The new login cannot be sa, and the user cannot be dbo, guest, or an INFORMATION_SCHEMA user.

sp_change_users_login cannot be used to map database users to Windows-level principals, certificates, or asymmetric keys.

sp_change_users_login cannot be used with a SQL Server login created from a Windows principal or with a user created by using CREATE USER WITHOUT LOGIN.

sp_change_users_login cannot be executed within a user-defined transaction.

Requires membership in the db_owner fixed database role. Only members of the sysadmin fixed server role can specify the Auto_Fix option.

A. Showing a report of the current user to login mappings

EXEC sp_change_users_login 'Report';

B. Mapping a database user to a new SQL Server login

--Create the new login.
CREATE LOGIN MaryB WITH PASSWORD = '982734snfdHHkjj3';
GO
--Map database user MB-Sales to login MaryB.
USE AdventureWorks;
GO
EXEC sp_change_users_login 'Update_One', 'MB-Sales', 'MaryB';
GO

C. Automatically mapping a user to a login, creating a new login if it is required

USE AdventureWorks;
GO
EXEC sp_change_users_login 'Auto_Fix', 'Mary', NULL, 'B3r12-3x$098f6';
GO

Reference

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