QBasic Tutorial 2

Q Basic Tutorial 2

QBasic Tutorial 1

Q Basic Tutorial 1

Three-Schema Architecture (3 view of DBMS)

 

Data Abstraction

Data abstraction is hiding the complex data structure in order to simplify the user’s interface of the system. It is done because many of the users interacting with the database system are not that much computer trained to understand the complex data structures of the database system.

To achieve data abstraction, we will discuss a Three-Schema architecture which abstracts the database at three levels discussed below:

 Three-Schema Architecture:

The main objective of this architecture is to have an effective separation between the user interface and the physical database. So, the user never has to be concerned regarding the internal storage of the database and it has a simplified interaction with the database system.

The three-schema architecture defines the view of data at three levels:

1. Physical level (internal level/view)
2. Logical level (conceptual level/view)
3. View level (external level/view)
   
   
   

1. Physical Level/ Internal Level

The physical or the internal level schema describes how the data is stored in the hardware. It also describes how the data can be accessed. The physical level shows the data abstraction at the lowest level and it has complex data structures. Only the database administrator operates at this level.

2. Logical Level/ Conceptual Level

It is a level above the physical level. Here, the data is stored in the form of the entity set, entities, their data types, the relationship among the entity sets, user operations performed to retrieve or modify the data and certain constraints on the data. Well adding constraints to the view of data adds the security. As users are restricted to access some particular parts of the database.

It is the developer and database administrator who operates at the logical or the conceptual level.

3. View Level/ User level/ External level 

It is the highest level of data abstraction and exhibits only a part of the whole database. It exhibits the data in which the user is interested. The view level can describe many views of the same data. Here, the user retrieves the information using different application from the database.

Schema in DBMS

The term "database schema" can refer to a visual representation of a database, a set of rules that govern a database, or to the entire set of objects belonging to a particular user. 

A database schema represents the logical configuration of all or part of a relational database. It can exist both as a visual representation and as a set of formulas known as integrity constraints that govern a database. These formulas are expressed in a data definition language, such as SQL. As part of a data dictionary, a database schema indicates how the entities that make up the database relate to one another, including tables, views, stored procedures, and more.

There are two main kinds of database schema:

1. A logical database schema conveys the logical constraints that apply to the stored data. It may define integrity constraints, views, and tables.
2. A physical database schema lays out how data is stored physically on a storage system in terms of files and indices.

 What is sub schema?

A sub schema is a subset of the schema and inherits the same property that a schema has. The plan (or scheme) for a view is often called sub schema. Sub schema refers to an application programmer's (user's) view of the data item types and record types, which he or she uses.