SQL Basics

Introduction to Databases (RDBMS) #

What is a Database? #

A database is a structured collection of data stored electronically.

Example:

• Customer records
• Sales data
• Employee data

What is RDBMS? #

RDBMS (Relational Database Management System) stores data in tables (rows & columns) and allows relationships between them.

Key Concepts #

Example Table #

What is SQL? #

SQL (Structured Query Language) is used to:

• Retrieve data
• Filter data
• Sort data
• Manage databases

SELECT Statement #

What is SELECT? #

Used to retrieve data from a table

Syntax #

SELECT column_name FROM table_name;

Examples #

Select all columns: #

SELECT * FROM Students;

Select specific columns:

SELECT Name, Age FROM Students;

FROM Clause #

What is FROM? #

Specifies which table to retrieve data from

Example #

SELECT Name FROM Students;

Students is the table

WHERE Clause #

What is WHERE? #

Used to filter data based on conditions

Syntax #

SELECT column FROM table
WHERE condition;

Examples #

Filter by age: #

SELECT * FROM Students
WHERE Age > 25;

Filter by text:

SELECT * FROM Students
WHERE City = 'London';

Operators #

DISTINCT #

What is DISTINCT? #

Removes duplicate values

Syntax #

SELECT DISTINCT column FROM table;

Example

SELECT DISTINCT City FROM Students;

Shows unique cities only

ORDER BY #

What is ORDER BY? #

Used to sort data

Syntax #

SELECT column FROM table
ORDER BY column ASC|DESC;

Examples #

Ascending (default): #

SELECT * FROM Students
ORDER BY Age ASC;

Descending:

SELECT * FROM Students
ORDER BY Age DESC;

LIMIT #

What is LIMIT? #

Used to restrict number of rows returned

Syntax #

SELECT column FROM table
LIMIT number;

8.3 Example

SELECT * FROM Students
LIMIT 3;

Returns first 3 rows

Combined Query Example #

SELECT DISTINCT City
FROM Students
WHERE Age > 25
ORDER BY City ASC
LIMIT 5;

This query:

• Removes duplicates
• Filters age > 25
• Sorts cities
• Returns top 5 results

Execution Order (Important) #

SQL runs in this order:

1. FROM
2. WHERE
3. SELECT
4. DISTINCT
5. ORDER BY
6. LIMIT

Table #

RDBMS → Stores data in tables SQL → Language to query data Core clauses:

• SELECT → retrieve
• FROM → source
• WHERE → filter
• DISTINCT → unique values
• ORDER BY → sort
• LIMIT → restrict results