Learn SQL (Best SQL Notes)

DBMS & SQL NOTES 

Database: A database is a collection of related data which represents some aspect of the real world. A database system is designed to be built and populated with data for a certain task. 

Database Management System (DBMS) is a software for storing and retrieving users' data while considering appropriate security measures. It consists of a group of programs which manipulate the database. The DBMS accepts the request for data from an application and instructs the operating system to provide the specific data. In large systems, a DBMS helps users and other third-party software to store and retrieve data. 

Database management systems were developed to handle the following difficulties of typical File-processing systems supported by conventional operating systems. 

1. Data redundancy and inconsistency 

2. Difficulty in accessing data 

3. Data isolation – multiple files and formats 

4. Integrity problems 

5. Atomicity of updates 

6. Concurrent access by multiple users 

7. Security problems 

ER diagram: 

● ER diagram or Entity Relationship diagram is a conceptual model that gives the graphical representation of the logical structure of the database. 

● It shows all the constraints and relationships that exist among the different components. 

● An ER diagram is mainly composed of the following three components- Entity Sets,  Attributes, and Relationship Sets.                    

● Roll_no is a primary key that can identify each entity uniquely.

Keys: 

A key is a set of attributes that can identify each tuple uniquely in the given relation. Types of Keys: 

● Super Key - A superkey is a set of attributes that can identify each tuple uniquely in the given relation. A super key may consist of any number of attributes. 

● Candidate Key - A set of the minimal attribute(s) that can identify each tuple uniquely in the given relation is called a candidate key. 

● Primary Key - A primary key is a candidate key that the database designer selects while designing the database. Primary Keys are unique and NOT NULL.