Principals (Database Engine)

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

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). This topic applies to all versions of SQL Server, but there are some restrictions on server-level principals in SQL Database or Azure Synapse Analytics.


Microsoft Entra ID was previously known as Azure Active Directory (Azure AD).

SQL Server-level principals

  • SQL Server authentication Login
  • Windows authentication login for a Windows user
  • Windows authentication login for a Windows group
  • Microsoft Entra authentication login for a Microsoft Entra user
  • Microsoft Entra authentication login for a Microsoft Entra group
  • Server Role

Database-level principals

  • Database User (There are 12 types of users. For more information, see CREATE USER.)
  • Database Role
  • Application Role

sa Login

The SQL Server sa login is a server-level principal. By default, it is created when an instance is installed. Beginning in SQL Server 2005 (9.x), the default database of sa is master. This is a change of behavior from earlier versions of SQL Server. The sa login is a member of the sysadmin fixed server-level role. The sa login has all permissions on the server and cannot be limited. The sa login cannot be dropped, but it can be disabled so that no one can use it.

dbo User and dbo Schema

The dbo user is a special user principal in each database. All SQL Server administrators, members of the sysadmin fixed server role, sa login, and owners of the database, enter databases as the dbo user. The dbo user has all permissions in the database and cannot be limited or dropped. dbo stands for database owner, but the dbouser account is not the same as the db_owner fixed database role, and the db_owner fixed database role is not the same as the user account that is recorded as the owner of the database.
The dbo user owns the dbo schema. The dbo schema is the default schema for all users, unless some other schema is specified. The dbo schema cannot be dropped.

public Server Role and Database Role

Every login belongs to the public fixed server role, and every database user belongs to the public database role. When a login or user has not been granted or denied specific permissions on a securable, the login or user inherits the permissions granted to public on that securable. The public fixed server role and the public fixed database role cannot be dropped. However you can revoke permissions from the public roles. There are many permissions that are assigned to the public roles by default. Most of these permissions are needed for routine operations in the database; the type of things that everyone should be able to do. Be careful when revoking permissions from the public login or user, as it will affect all logins/users. Generally you should not deny permissions to public, because the deny statement overrides any grant statements you might make to individuals.

INFORMATION_SCHEMA and sys Users and Schemas

Every database includes two entities that appear as users in catalog views: INFORMATION_SCHEMA and sys. These entities are required for internal use by the Database Engine. They cannot be modified or dropped.

Certificate-based SQL Server Logins

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.

  • ##MS_SQLResourceSigningCertificate##
  • ##MS_SQLReplicationSigningCertificate##
  • ##MS_SQLAuthenticatorCertificate##
  • ##MS_AgentSigningCertificate##
  • ##MS_PolicyEventProcessingLogin##
  • ##MS_PolicySigningCertificate##
  • ##MS_PolicyTsqlExecutionLogin##

These principal accounts do not have passwords that can be changed by administrators as they are based on certificates issued to Microsoft.

The guest User

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 its CONNECT permission. The CONNECT permission can be revoked by executing REVOKE CONNECT FROM GUEST; within any database other than master or tempdb.

For information about designing a permissions system, see Getting Started with Database Engine Permissions.

The following topics are included in this section of SQL Server Books Online:

See Also

Securing SQL Server
sys.database_principals (Transact-SQL)
sys.server_principals (Transact-SQL)
sys.sql_logins (Transact-SQL)
sys.database_role_members (Transact-SQL)
Server-Level Roles
Database-Level Roles