Schema and Instance in DBMS
12 Apr Admin
Schema
A schema is the complete design of database it is also known as intension. It is the collection of named objects.
Design of a database is called the schema. Schema is of three types: Physical schema, logical schema and view schema.
The database system has various schemata separated according to the levels of abstraction :-
- The physical schema is the lowest level of a schema which describes how the data stored on the disk or the physical storage.
- The logical schema is the intermediate level of a schema which describes the structure of the database to the database designers. It also specifies what relationship exists between the data.
- The external schema or subschema is the highest level of a schema which defines the views for the end users.
Instance
An instance is the information collected in a database at some specific moment, and it is also known as state or extension. It is a snapshot where the current state or occurrence of a database is framed at that moment.
The data stored in database at a particular moment of time is called instance of database. Database schema defines the variable declarations in tables that belong to a particular database; the value of these variables at a moment of time is called the instance of that database.
Schema and instance are somehow related, a schema is the initial state of the database where the database is designed at first. On the other hand, an instance is a state when data is loaded into the database or when any change is acquired by the corresponding database. Schema is the detailed description of the structure of database whereas the information stored at a specific moment in a database is known as an instance.
Data Abstraction in DBMS
03 Sep Admin
Abstraction,in general, is the process of taking away or removing characteristics from something in order to reduce it to a set of essential characteristics.Database systems are made-up of complex data structures. To ease the user interaction with database, the developers hide internal irrelevant details from users. This process of hiding irrelevant details from user is called data abstraction.
There are mainly 3 levels of data abstraction:
Physical level: Physical level is the lowest level of data abstraction which describes how data is actually stored in database. You can get the complex data structure details at this level.
Logical level: Logical level is the middle level of 3-level data abstraction architecture which describes what data is stored in database.
View level: Highest level of data abstraction is view level which describes the user interaction with database system.
Levels of DBMS Architecture
03 Sep Admin
This architecture has three levels:
1. External level :-
External level is also called view level. The reason this level is called “view” is because several users can view their desired data from this level which is internally fetched from database with the help of conceptual and internal level mapping. External level is the “ top level ” of the Level of DBMS Architecture.
Facts about External level:
- An external level is only related to the data which is viewed by specific end users.
- This level includes some external schemas.
- External schema level is nearest to the user
- The external schema describes the segment of the database which is needed for a certain user group and hides the remaining details from the database from the specific user group
2. Conceptual level :-
Conceptual level is also called logical level .The whole design of the database such as relationship among data, schema of data etc. are described in this level. It is maintained by DBA.
Facts about Conceptual level:
- Defines all database entities, their attributes, and their relationships
- Security and integrity information
- In the conceptual level, the data available to a user must be contained in or derivable from the physical level
3. Internal level :-
Internal level is also known as physical level. This level describes how the data is actually stored in the storage devices. It is responsible for storage allocation. This is the lowest level of the architecture.
Facts about Internal level:
- The internal schema is the lowest level of data abstraction
- It helps you to keeps information about the actual representation of the entire database. Like the actual storage of the data on the disk in the form of records
- The internal view tells us what data is stored in the database and how
- It never deals with the physical devices. Instead, internal schema views a physical device as a collection of physical pages.
Views of data
03 Sep Admin
Different Views of Database
There are three different views of the database. The views are seen through the schema and the sub-schema of the data.
Logical view of the user of data
This is a view of the application programmer or user as he wants it. This could be a partial view of the database. For example, the quality control manager and the purchase manager would view the data differently for their use.
The view of database administrator (DBA)
This is a view of the person who is managing the database through the creation, updating, structuring and ensuring that it is up-to-date. The view of the DBA is a global view.
Physical View
This is the view of the data actually stored and organized on the physical devices.
Architecture of DBMS
03 Sep Admin
DBMS architecture will help us understand the components of database system and the relation among them. The architecture of DBMS depends on the computer system on which it runs.
Types of DBMS Architecture
There are three types of DBMS architecture:
1. Single tier architecture :-
In this type of architecture, the database is readily available on the client machine, any request made by client doesn’t require a network connection to perform the action on the database.
2. Two tier architecture :-
In two-tier architecture, the Database system is present at the server machine and the DBMS application is present at the client machine, these two machines are connected with each other through a reliable network.Whenever client machine makes a request to access the database present at server using a query language like sql, the server perform the request on the database and returns the result back to the client.
3. Three tier architecture :-
In this architecture, another layer is present between the client machine and server machine. In this architecture, the client application doesn’t communicate directly with the database systems present at the server machine, rather the client application communicates with server application and the server application internally communicates with the database system present at the server.
Advantages of DBMS Over File System
03 Sep Admin
DBMS vs. File System :-
| DBMS | File Management System |
| Multi-user access | It does not support multi-user access |
| Design to fulfill the need for small and large businesses | It is only limited to smaller DBMS system. |
| Remove redundancy and Integrity | Redundancy and Integrity issues |
| Expensive. But in the long term Total Cost of Ownership is cheap | It’s cheaper |
| Easy to implement complicated transactions | No support for complicated transactions |
Database Applications
03 Sep Admin
Application Of DBMS :-
| Sector | Use of DBMS |
| Banking | For customer information, account activities, payments, deposits, loans, etc. |
| Airlines | For reservations and schedule information. |
| Universities | For student information, course registrations, colleges and grades. |
| Telecommunication | It helps to keep call records, monthly bills, maintaining balances, etc. |
| Finance | For storing information about stock, sales, and purchases of financial instruments like stocks and bonds. |
| Sales | Use for storing customer, product & sales information. |
| Manufacturing | It is used for the management of supply chain and for tracking production of items. Inventories status in warehouses. |
| HR Management | For information about employees, salaries, payroll, deduction, generation of paychecks, etc. |
Introduction of DBMS
03 Sep Admin
- Introduction of DBMS
DBMS stands for Database Management System. We can break it like this DBMS = Database + Management System. Database is a collection of data and Management System is a set of programs to store and retrieve those data. Based on this we can define DBMS like this: DBMS is a collection of inter-related data and set of programs to store & access those data in an easy and effective manner.
Characteristics of DBMS
- Provides security and removes redundancy
- Self-describing nature of a database system
- Insulation between programs and data abstraction
- Support of multiple views of the data
- Sharing of data and multiuser transaction processing
- DBMS allows entities and relations among them to form tables.
- It follows the ACID concept ( Atomicity, Consistency, Isolation, and Durability).
- DBMS supports multi-user environment that allows users to access and manipulate data in parallel.
What is the need of DBMS?
Database systems are basically developed for large amount of data. When dealing with huge amount of data, there are two things that require optimization: Storage of data and retrieval of data.
Purpose of Database Systems
The main purpose of database systems is to manage the data. To manage the data we need to store this data somewhere where we can add new data, delete unused data, update outdated data, retrieve data.
Links
03 Sep Admin
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