In a database the structure of a database is a complex and it is made easy to the user to access data form a database, in a database the developers hide the internal irrelevant details from users. The process of hiding the irrelevant data from user is called Database abstraction.
Level of Data Abstraction
There are Two level of data abstractions:
- Physical Level:
- It is a lowest level of data abstraction.
- It describes the how the data is actual stored disk
- It is also called internal level.
- The internal view described by the internal schema.
- Logical Level:
- It is also called conceptual level.
- Conceptual level lies between view level and physical level
- It describes what data are stored in a database and what relationships exits among these data.
- The conceptual database schema is describes by relation.
- View Level:
- It is also called external level or user level.
- It is the top of data abstraction.
- This level is concerned with users and it describes the part of database that a particular user is interested in and hides the rest of the database from user.
The capacity to change the conceptual schema without affecting the applications program is called data independent. There are two kinds of data independent:
- Logical data independent: the capacity of change the conceptual schema without affecting the application program is called logical data independent.
- Physical Data independent: The capacity of change the internal schema without affecting the application program is called physical data independent.
A DBA is a person who has central control both data and programs. The responsibilities of Database Administrator are as follows:
- Schema definition and modification.
- New software installation.
- Security enforcement and administration.
- Data Analysis.
- Physical database design.
- Routine management
- Physical organization modification.
- Give the backup and recovery strategies.
Types of Relationships:
It is the association among entities for a binary relationship set R between entity set A and set B the relationship may be
One to one (1:1): An entity A as associated with at most one entity. In B and N entity in B is associated with at most one entity in A. eg principle and college
One to Many (1:M): An entity in a associated with any number of entity an entity in B how ever B can be associated with at most one entity in A. eg. Teacher and Course
- Many to many (m:m): An entity in A associated with any number of entity in B in entity in B. in entity B is associated with any number of entity in A. eg. Student and Course.