Instances

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You can have multiple independent copies of SQL Server running simultaneously on the same computer. Each of these is called an instance of SQL Server. Each instance maintains its own separate set of database files.

If you already have SQL Server running at your site, it presumably manages a set of databases. Your IT department may prefer to keep your MainBoss database(s) separate from databases for other software packages. You can do that by creating a new instance of the SQL Server. This can be set up so that it only deals with MainBoss databases.

Different instances of SQL Server are distinguished by having different instance names. This is just an identifying name. One instance can have its name blank, in which case it's called the default instance. If someone creates a SQL Server database on a computer without specifying an instance name, the database is created and managed by the default instance.

By default, the installation procedures for full versions of SQL Server set up an instance with its name blank. However, for SQL Express, the default installation procedure specifies the instance name SQLEXPRESS.

Note: Don't create multiple instances of SQL Server unless you have a good reason. Running multiple instances introduces complexity; it can lead to confusion, inconsistency and human error in how SQL Server is managed. In addition, the computer will probably run more slowly.

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