Data Abstraction and Data Independence

Database systems comprise complex data structures. In order to make the system efficient in terms of retrieval of data, and reduce complexity in terms of usability of users, developers use abstraction i.e. hide irrelevant details from the users. This approach simplifies database design. 

Level of Abstraction in a DBMS

There are mainly 3 levels of data abstraction: 

• Physical or Internal Level
• Logical or Conceptual Level
• View or External Level

Physical or Internal Level

This is the lowest level of data abstraction. It tells us how the data is actually stored in memory. Access methods like sequential or random access and file organization methods like B+ trees and hashing are used for the same. Usability, size of memory, and the number of times the records are factors that we need to know while designing the database. 
Suppose we need to store the details of an employee. Blocks of storage and the amount of memory used for these purposes are kept hidden from the user. 

Logical or Conceptual Level

This level comprises the information that is actually stored in the database in the form of tables. It also stores the relationship among the data entities in relatively simple structures. At this level, the information available to the user at the view level is unknown. 
We can store the various attributes of an employee and relationships, e.g. with the manager can also be stored. 

View or External Level

This is the highest level of abstraction. Only a part of the actual database is viewed by the users. This level exists to ease the accessibility of the database by an individual user. Users view data in the form of rows and columns. Tables and relations are used to store data. Multiple views of the same database may exist. Users can just view the data and interact with the database, storage and implementation details are hidden from them. 

Example: In case of storing customer data,

• Physical level – it will contains block of storages (bytes,GB,TB,etc)
• Logical level –  it will contain the fields and the attributes of data.
• View level – it works with CLI or GUI access of database

Data Abstraction

The main purpose of data abstraction is to achieve data independence in order to save the time and cost required when the database is modified or altered. 

Data Independence

Data Independence is mainly defined as a property of DBMS that helps you to change the database schema at one level of a system without requiring to change the schema at the next level. it helps to keep the data separated from all program that makes use of it.
We have namely two levels of data independence arising from these levels of abstraction: 

• Physical level data independence
• Logical level data independence

Data Independence

Physical Level Data Independence

It refers to the characteristic of being able to modify the physical schema without any alterations to the conceptual or logical schema, done for optimization purposes, e.g., the Conceptual structure of the database would not be affected by any change in storage size of the database system server. Changing from sequential to random access files is one such example. These alterations or modifications to the physical structure may include: 

• Utilizing new storage devices.
• Modifying data structures used for storage.
• Altering indexes or using alternative file organization techniques etc.

Logical Level Data Independence

It refers characteristic of being able to modify the logical schema without affecting the external schema or application program. The user view of the data would not be affected by any changes to the conceptual view of the data. These changes may include insertion or deletion of attributes, altering table structures entities or relationships to the logical schema, etc.