Oracle is a Relational Database Management System. It was designed to make it possible for allowing simultaneous access to a larger amount of stored data. Normally RDBMS consists of the database (this contains the physical files residing in the system and also the logical data) and the instance. The Oracle database has two layers.

The Physical Layer – This layer consists of files like datafiles, redo log files and control files that reside on the disk. Datafiles consists of information contained in the database. Redo log files are used to hold information which can be used during recovery in the event of a system failure. Control files contain information used to start an instance.

The Logical Layer – This layer consists of one or more tablespaces (tablespace is used to group data together logically) and the database schema (database schema is a collection of logical structure objects) consisting of various items such as tables, clusters, etc.

The database schema consists of:

·         Table – It is the basic storage unit in the Oracle database.

·         View – It acts as a window into one or more tables. A view, like a table, can be updated, deleted and queried.

·         Cluster – It is a group of tables stored together as one table sharing a common column.

·         Index – It is used to retrieve data more quickly and efficiently.

·         Stored Procedure – It is a predefined query that is stored in the data dictionary.

·         Sequence – It is used to generate a unique sequence of numbers in a cache.

·         Database Trigger – It is a procedure that is invoked automatically when an event occurs.