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SQL & Database Basics

  • Learn the fundamentals of SQL and relational databases.

  • Database Concepts

    What is a Database?

    A Database is an organized collection of data stored electronically.

    It helps to:

    • Store data

    • Manage data

    • Retrieve data

    • Update data


    What is DBMS?

    DBMS (Database Management System) is software used to manage databases.

    Examples:

    • MySQL

    • PostgreSQL

    • MongoDB

    • Oracle Database


    Types of Databases

    Relational Database (RDBMS)

    • Data stored in tables

    • Uses SQL

    • Example: MySQL

    NoSQL Database

    • Flexible structure

    • Stores JSON-like data

    • Example: MongoDB


    Tables

    A Table is used to store data in rows and columns.

    Structure:

    ID

    Name

    Subject

    Marks

    1

    Rahul

    Math

    85

    2

    Priya

    Science

    90

    • Columns → Attributes (ID, Name, Subject)

    • Rows → Records


    Create Table Example:

Creating a Students Table in SQL

This SQL query creates a table named students with columns for id, name, subject, and marks to store student information and their marks. 

CREATE TABLE students (
    id INT,
    name VARCHAR(50),
    subject VARCHAR(50),
    marks INT
);
  • Insert Data:

Inserting Data into Students Table

This SQL query inserts a new record into the students table with student ID 1, name Rahul, subject Math, and marks 85. 

INSERT INTO students VALUES (1, 'Rahul', 'Math', 85);

  • Primary Key

    What is Primary Key?

    A Primary Key is a column that:

    • Uniquely identifies each record

    • Cannot be NULL

    • Cannot have duplicate values

    Example:

Primary Key in SQL

This example explains how to define a Primary Key in a SQL table. A Primary Key uniquely identifies each record and does not allow NULL or duplicate values. 

CREATE TABLE students (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    subject VARCHAR(50),
    marks INT
);

  • Here:

    • id is Primary Key

    • No two students can have same ID

    Why Primary Key is Important?

    ✔ Prevents duplicate records
     ✔ Maintains data integrity
     ✔ Improves query performance
     ✔ Helps create relationships between tables


    Real-World Example

    In Banking System:

    Account_Number

    Customer_Name

    Balance

    Account_Number is Primary Key
     Each account must be unique.

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