SQL Server supports multiple instances of SQL Server on a single server or processor, but only one instance can be the default instance. All others must be named instances. A computer can run multiple instances of SQL Server concurrently, and each instance runs independently of other instances.

For more information, see Maximum Capacity Specifications for SQL Server 2008.

Failover cluster instances only — Specify the SQL Server failover cluster network name. This name identifies the failover cluster instance on the network.

Default or Named instance — Consider the following information when you decide whether to install a default or named instance of SQL Server:

• If you plan to install a single instance of SQL Server on a database server, it should be a default instance.
  
• Use a named instance for situations where you plan to have multiple instances on the same computer. A server can host only one default instance.
  
• Any application that installs SQL Server Express should install it as a named instance. This will minimizes conflict when multiple applications are installed on the same computer.
  

Default instance

Select this option to install a default instance of SQL Server. A computer can host only one default instance; all other instances must be named. However, if you have a default instance of SQL Server installed, you can add a default instance of Analysis Services to the same computer.

Named instance

Select this option to create a new named instance. Be aware of the following when you name an instance of SQL Server:

• Instance names are not case sensitive.
  
• Instance names cannot contain the term "Default" or other reserved keywords. If a reserved keyword is used in an instance name, a Setup error will occur. For more information, see Reserved Keywords (Setup).
  
• If you specify MSSQLServer for the instance name, a default instance will be created. For SQL Server Express, if you specify SQLExpress for the instance name, a default instance will be created.
  
• Instance names are limited to 16 characters.
  
• The first character in the instance name must be a letter. Acceptable letters are those defined by the Unicode Standard 2.0. These include Latin characters a-z, A-Z, and letter characters from other languages.
  
• Subsequent characters can be letters defined by the Unicode Standard 2.0, decimal numbers from Basic Latin or other national scripts, the dollar sign ($), or an underscore (_).
  
• Embedded spaces or other special characters are not allowed in instance names. The backslash (\), comma (,), colon (:), semi-colon (;), single quote ('), ampersand (&), and at sign (@) are also not allowed.
  

  Note:
  Only characters that are valid in the current Windows code page can be used in SQL Server instance names. If an unsupported Unicode character is used, a Setup error will occur.

Detected instances and features

View a list of installed SQL Server instances and components on the computer where SQL Server Setup is running.

Instance ID - By default, the instance name is used as the Instance ID. This is used to identify installation directories and registry keys for your instance of SQL Server. This is the case for default instances and named instances. For a default instance, the instance name and instance ID would be MSSQLSERVER. To use a non-default instance ID, specify it in the Instance ID field.

Note:
Instance ID's that begin with an underscore (_) or that contain the number sign (#) or the dollar sign ($) are not supported.

For more information about directories, file locations, and instance ID naming, see File Locations for Default and Named Instances of SQL Server.

Instance root directory - By default, the instance root directory is C:\Program Files\Microsoft SQL Server\. To specify a non-default root directory, click the Browse button, or provide a path name.

All components of a given instance of SQL Server are managed as a unit. All SQL Server service packs and upgrades will apply to every component of an instance of SQL Server.

All components of SQL Server that share the same instance name must meet the following criteria:

• Same version
  
• Same edition
  
• Same language settings
  
• Same clustered state
  

  Note:
  Reporting Services is not cluster-aware.

• Same operating system
  

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