When I install SQL Server 2008 Express in prompts me to create an instance and aborts if I don't. Then I see that information in an entry in Sql Server Configuration Manager on SQL Server Services. What is a SQL Server instance?
An SQL Server instance is a complete SQL server and you can install many instances on a machine but you can have only 1 default instance.
An SQL Server instance has its own copy of the server files, databases and security credentials.
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SQL SERVER INSTANCE An instance of the Database Engine is a copy of the sqlservr.exe executable that runs as an operating system service. Each instance manages several system databases and one or more user databases. Each computer can run multiple instances of the Database Engine independently of other instances.
SQL Server is composed of three main parts: 1. An engine, which is the software started by a few Windows services that performs lookups, sorts, and other actions; 2. Meta-data such as the master and msdb system databases; 3. User databases where data is stored.
Instances also allow you to work with multiple versions and even editions of the product. You can install Instances at any time, even after SQL Server is installed and running for some time. So for Instance (no pun intended), you can install SQL Server 2005 Express Edition, SQL Server 2005 Enterprise Edition, SQL Server 2008 Standard Edition and SQL Server 2008 R2 Developer Edition, all on the same hardware.
Instances apply primarily to the database engine and its supporting components, not to the client tools. When you install multiple instances, each instance gets a unique set of: 1. System and user databases. 2. The SQL Server and SQL Server Agent services. For default instances, the names of the services remain MSSQLServer and SQLServerAgent. For named instances, the names of the services are changed to MSSQL$instancename and SQLAgent$instancename, allowing them to be started and stopped independently of the other instances on the server. The database engines for the different instances are started and stopped using the associated SQL Server service. The SQL Server Agent services manage scheduled events for the associated instances of the database engine. 3. The registry keys associated with the database engine and the SQL Server and SQL Server Agent services separate versions and releases.. 4. Network connection addresses so that applications can connect to specific instances.