Principals (Database Engine)
Principals are entities that can request SQL Server resources. Like other components of the SQL Server authorization model, principals can be arranged in a hierarchy. The scope of influence of a principal depends on the scope of the definition of the principal: Windows, server, database; and whether the principal is indivisible or a collection. A Windows Login is an example of an indivisible principal, and a Windows Group is an example of a principal that is a collection. Every principal has a security identifier (SID).
Windows Domain Login
Windows Local Login
SQL Server-level principals
SQL Server Login
The SQL Server sa log in is a server-level principal. By default, it is created when an instance is installed. Beginning in SQL Server 2005, the default database of sa is master. This is a change of behavior from earlier versions of SQL Server.
Every database user belongs to the public database role. When a user has not been granted or denied specific permissions on a securable, the user inherits the permissions granted to public on that securable.
Every database includes two entities that appear as users in catalog views: INFORMATION_SCHEMA and sys. These entities are required by SQL Server. They are not principals, and they cannot be modified or dropped.
Server principals with names enclosed by double hash marks (##) are for internal system use only. The following principals are created from certificates when SQL Server is installed, and should not be deleted.
Each database includes a guest. Permissions granted to the guest user are inherited by users who have access to the database, but who do not have a user account in the database. The guest user cannot be dropped, but it can be disabled by revoking it's CONNECT permission. The CONNECT permission can be revoked by executing REVOKE CONNECT FROM GUEST within any database other than master or tempdb.
By definition, a client and a database server are security principals and can be secured. These entities can be mutually authenticated before a secure network connection is established. SQL Server supports the Kerberos authentication protocol, which defines how clients interact with a network authentication service.