What is different between database and instance?

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A database is a collection of three important files which include data files, control files and redo log files which physically exist on a disk. A database may be mounted and opened by one or more instances (using RAC). 

An instance is a combination of oracle background process (SMON, PMON, DBWR, LGWR) and memory structure (SGA, PGA). An instance can mount and open only a single database, ever.


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