Named , Default and Multiple Instances of SQL Server

When you install SQL Server, the Microsoft SQL Server Installation Wizard gives you the option of defining the installation as the default instance or as a named instance.

 A named instance simply means that you define a mane for the instance during the installation. You will then have to access that instance by name.

Default instances acquire the name of the server you install them on. Thus you can have only one default instance at a time, but you can have mane name instance.

 When you start the Microsoft SQL Server Installation Wizard by running the setup.exe application, it detects whether a default instance already exists on the computer. If the wizard does not detect a default instance, it gives you the choice of installing a default instance or a named instance, clear the Default check box and type in the named instance.

Using multiple instances of SQL Server increase administration overhead and causes duplication of components. Additional instances of the SQL Server and SQL Server Agent services require additional computer resources: memory and processing capacity

In the following scenarios, multiple instances of SQL Server can be used:

• While testing multiple version of SQL Server on the one computer.
• While testing service packs and development databases and applications.
• While different customers require their own system and user databases along with full administrative control of their SQL instance.
• While the desktop engine is embedded in applications because each application can install its own instance independent of instances installed by other applications.