SQL Server Express User Instances

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Microsoft SQL Server Express Edition (SQL Server Express) supports the user instance feature, which is only available when using the .NET Framework Data Provider for SQL Server (SqlClient). A user instance is a separate instance of the SQL Server Express Database Engine that is generated by a parent instance. User instances allow users who are not administrators on their local computers to attach and connect to SQL Server Express databases. Each instance runs under the security context of the individual user, on a one-instance-per-user basis.

User Instance Capabilities

User instances are useful for users who are running Windows under a least-privilege user account (LUA). Each user has SQL Server system administrator (sysadmin) privileges over the instance running on their computer without needing to run as a Windows administrator as well. Software executing on a user instance with limited permissions cannot make system-wide changes because the instance of SQL Server Express is running under the non-administrator Windows account of the user, not as a service. Each user instance is isolated from its parent instance and from any other user instances running on the same computer. Databases running on a user instance are opened in single-user mode only, and it is not possible for multiple users to connect to databases running on a user instance. Replication and distributed queries are also disabled for user instances.

Enabling User Instances

To generate user instances, a parent instance of SQL Server Express must be running. User instances are enabled by default when SQL Server Express is installed, and they can be explicitly enabled or disabled by a system administrator executing the sp_configure system stored procedure on the parent instance.

SQL

-- Enable user instances.  
sp_configure 'user instances enabled','1'
  
-- Disable user instances.  
sp_configure 'user instances enabled','0'  

The network protocol for user instances must be local Named Pipes. A user instance cannot be started on a remote instance of SQL Server, and SQL Server logins are not allowed.