Friday, 10 February 2017

What is data abstraction and levels of data abstraction

What is data abstraction and levels of data abstraction, define data abstraction, what is physical level or internal level in dbms, what is logical level or conceptual level in dbms, what is view level or external level in dbms

Data Abstraction

It is one of the main and important characteristics of database approach. Data abstraction is the concept of hiding the details like data definition, data organization and storage of data from the end users and showing them only the essential things as per their requirement.
For example, an end user may be naïve user, application programmer, or an expert in DBMS. Their requirement in viewing data is different for different user. The users may not be interested in everything about a database. Since most of the end users of any database may not have enough idea about the implementation, the database developers have hidden many of the unwanted (for end users) information through several levels of data abstraction. They are;

Physical level – it is the lowest level of data abstraction dealing the low-level data structures. This level answers the following How questions;

  • How a table is stored?
  • How data are arranged in a table?
  • How each and every record of a table is kept together?
  • How tables are stored in the disk? etc.
In physical level the developers define the following;

  • Storage space allocation for data and indexes;
  • Record descriptions for storage (with stored sizes for data items);
  • Record placement;
  • Data compression and data encryption techniques

Logical level (conceptual level) – the next higher level of data abstraction is the logical level. It deals with the structure of tables and the relationships between the tables. It answers all the following What questions;

  • What are the attributes of a table?
  • What are the records stored in a table?
  • What index is used to access a table?
  • How many indexes are there? etc.
In logical level we define the following;
  • All entities, their attributes, and their relationships;
  • The constraints on the data;
  • Semantic information about the data;
  • Security and integrity information.

View level (external level) – the highest level of data abstraction is view level. It answers the different users’ needs with the simplified interaction with the system. It defines the following;
  • The attributes of a table that can be shown to a particular user or particular type of user;
  • The data stored in a table that could be shown to a particular type of user;
  • The operation a user can perform; etc.


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