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Principals (Database Engine)

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW) SQL database in Microsoft Fabric Preview

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.

Note

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
  • Microsoft Entra authentication login for a Microsoft Entra service principal
  • Server Role

Database-level principals

sa Login

The SQL Server sa login is a server-level principal. By default, it's 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 can't be limited. The sa login can't 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 can't be limited or dropped. dbo stands for database owner, but the dbo user account isn't the same as the db_owner fixed database role, and the db_owner fixed database role isn't 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 can't 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 hasn't 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 can't 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 shouldn't 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 can't 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 shouldn't be deleted.

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

These principal accounts don't have passwords that can be changed by administrators as they're 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 don't have a user account in the database. The guest user can't 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.

Limitations

  • In SQL database in Microsoft Fabric Preview, only database-level users and roles are supported. Server-level logins, roles, and the sa account aren't available. In SQL database in Microsoft Fabric Preview, Microsoft Entra ID for database users is the only supported authentication method. For more information, see Authorization in SQL database in Microsoft Fabric.

For information about designing a permissions system, see Get started with Database Engine permissions.

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

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