SQL Server Instance and its types

A SQL Server instance is a separate installation of SQL Server on a single machine. Each instance of SQL Server is a separate instance of the SQL Server software, with its own set of system databases, configuration settings, and user databases.

There are two types of SQL Server instances:

Default Instance: this is the default installation of SQL Server and is identified by its instance name, which is simply "MSSQLSERVER". Only one default instance can be installed on a machine. For example, if you have installed SQL Server on your machine, the default instance would be accessible by using the server name "localhost" or "127.0.0.1" and the instance name "MSSQLSERVER".

Named Instance: a named instance is a separate installation of SQL Server on the same machine as a default instance. Named instances are identified by their instance names, which are specified during the installation process. Multiple named instances can be installed on a single machine. For example, if you have a second instance of SQL Server installed on the same machine as the default instance, you might specify the instance name "SQLSERVER2". The named instance would be accessible by using the server name "localhost" or "127.0.0.1" and the instance name "SQLSERVER2".


Using named instances can be useful in scenarios where multiple applications or services on the same machine require separate instances of SQL Server, each with its own configuration, databases, and security settings. For example, you might have one instance of SQL Server for your development environment and another instance for your production environment, each with its own configuration, security settings, and databases. Named instances also provide a way to run multiple versions of SQL Server on the same machine. For example, you could have one instance of SQL Server 2017 and another instance of SQL Server 2019 on the same machine, each with its own databases and configuration settings.

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