「First, you can start a database instance without having it accessing any database files. This is how you create a database, starting an instance first and creating the database from within the instance. Second, an instance can access only one database at a time. When you start an instance, …」
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An Overview of Oracle Database Architecture
https://www.oracle.com/webfolder/technetwork/tutorials/architecture-diagrams/18/technical-architecture/database-technical-architecture.html
