External Level :
- The User’s view of the database
- Consists of a no. of different external views of the DB
- Describes part of the DB for particular group of users
- Provides a powerful & flexible security mechanism by hiding part of DB from certain users. The user is not aware of any existing attributes that are missing from the view.
- It permits users to access data in a way that is customized to their needs, so that the same data can be seen by different users in ways, at the same time.
Objective: Separate each user’s view of database from way it is physically represented.
Conceptual View
- The Logical Data Structure of the entire database as seen by DBA
- What data is stored in database
- The relationships among the data
- Complete view of the data requirements of the organization, independent of any storage consideration
Represents
- Entities, Attributes, Relations
- Constraints on data
- Semantic info. on data
- Security, Integrity info.
Supports each external view: any data available to a user must be contained in, or derivable from the conceptual level.
External Level
- Physical representation of the DB on the comp.
- How the data is stored in database
- Physical implementation of the DB to achieve optimal run-time performance & storage space utilization
Storage space allocation for data & indexes
Record description for storage
Record placement
Data Compression, Encryption
Physical Level
Managed by the OS under the direction of the DBMS.
