INTRODUCTION TO SQL

Agenda – Session 1: Introduction to SQL

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• What is SQL?
• Types of SQL Commands
• Creating a Database and Table
• Core SQL Statements

SELECT

INSERT

UPDATE

DELETE

• Filtering and Sorting Data

WHERE Clause

ORDER BY

LIMIT

Aliasing (AS)

• Grouping and Aggregation

GROUP BY

HAVING vs WHERE

• Q&A and Wrap-Up

WHAT IS SQL

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SQL stands for Structured Query Language.

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It is used to communicate with databases. SQL is used to create, read, update, and delete data (commonly called CRUD operations).

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SQL works with Relational Databases like MySQL, PostgreSQL, SQL Server, Oracle, etc.

WHY SQL IS IMPORTANT

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Data is everywhere – apps, websites, banks, hospitals, social media – all run on databases.

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SQL helps you interact with that data in a structured and powerful way.

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It’s not just for developers — data analysts, testers, business analysts, and even product managers use SQL every day.

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SQL is a top in-demand skill — often tested in interviews for internships and jobs.

1. DDL – Data Definition Language

Used to define or modify the structure of database objects (tables, schemas, etc.).

CREATE – Create a new table or database

ALTER – Modify the structure of an existing table

DROP – Delete a table or database

TYPES OF SQL COMMANDS

2. DML – Data Manipulation Language

Used to manage data within tables (insert, update, delete records).

INSERT – Add new records

UPDATE – Modify existing records

DELETE – Remove records

3. DCL – Data Control Language

Used to control access and permissions in the database.

GRANT – Provide access rights

REVOKE – Remove access rights

4. TCL – Transaction Control Language

Used to manage transactions in the database.

COMMIT – Save changes permanently

ROLLBACK – Undo changes

SAVEPOINT – Mark a point in a transaction for partial rollback

Creating and Populating a Table in SQL

1. Creating a Database

CREATE DATABASE college;

USE college;

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2. Creating a Table

CREATE TABLE students (

id INT PRIMARY KEY,

name VARCHAR(50),

age INT,

grade CHAR(2)

);

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3. Inserting Data into the Table

INSERT INTO students (id, name, age, grade)

VALUES (1, 'Ananya', 20, 'A');

INSERT INTO students (id, name, age, grade)

VALUES (2, 'Rahul', 21, 'B’);

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Explanation:

INSERT INTO specifies the table and columns.

VALUES provides the actual data in the same order as the columns.

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INSERT INTO students (id, name, age, grade)

VALUES

(3, 'Sneha', 19, 'A'),

(4, 'Vikram', 22, 'C');

CORE SQL COMMANDS

Important Tip: Always use a WHERE clause with UPDATE and DELETE unless you intend to change/delete all records.

What Happens If You Don’t Use WHERE?

If you omit the WHERE clause in UPDATE or DELETE, the command will affect all rows in the table. This is one of the most common and dangerous mistakes in SQL.

Filtering Data with WHERE Clause

The WHERE clause is used to filter records based on a specific condition.

Syntax:

SELECT column1, column2

FROM table_name

WHERE condition;

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Example:

SELECT name, grade

FROM students

WHERE grade = 'A';

Retrieves only students who scored an 'A'.

Common Operators Used in WHERE:

= (equal)

!= or <> (not equal)

> , < , >=, <=

BETWEEN, IN, LIKE, IS NULL

Refining Query Results with ORDER BY and LIMIT

Common Use Together:

SELECT name, grade FROM students ORDER BY grade DESC LIMIT 2;

Returns the top 2 students with the highest grades

Simplifying Output with SQL Aliases

What is Aliasing in SQL?

Aliasing is used to rename columns or tables temporarily to make results more readable or concise, especially in complex queries.

Column Alias Syntax:

SELECT column_name AS alias_name

FROM table_name;

Example:

SELECT name AS student_name, grade AS final_grade

FROM students;

Renames name to student_name and grade to final_grade in the result.

Table Aliasing in SQL

Query:

SELECT t.name

FROM students AS t;

students is the original table.

t is an alias (temporary name) given to students.

We then use t to refer to the table instead of writing students again.

Adding Two Columns

Suppose you have a table called salary_info:

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emp_id base_salary bonus

101 40000 5000

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Query:

SELECT emp_id, base_salary + bonus AS total_salary

FROM salary_info;

This adds base_salary and bonus and returns it as total_salary.

Example with JOIN:

SELECT s.name, d.department_name

FROM students AS s

JOIN departments AS d ON s.dept_id = d.dept_id;

Here, s and d are table aliases for readability.

Using GROUP BY and HAVING Clauses in SQL

GROUP BY – Group Rows with Similar Values

GROUP BY is used with aggregate functions like COUNT(), SUM(), AVG(), etc., to group rows based on one or more columns.

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Syntax:

SELECT column, AGGREGATE_FUNCTION(column)

FROM table_name

GROUP BY column;

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Example:

SELECT grade, COUNT(*) AS count_of_students

FROM students

GROUP BY grade;

Groups students by grade and counts how many are in each grade.

HAVING – Filter Groups After Aggregation

Unlike WHERE, which filters before grouping, HAVING filters after the group results are generated.

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Syntax:

SELECT column, AGGREGATE_FUNCTION(column)

FROM table_name

GROUP BY column

HAVING condition;

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Example:

SELECT grade, COUNT(*) AS count_of_students

FROM students

GROUP BY grade

HAVING COUNT(*) > 1;

Displays only the grades that appear more than once.

WHERE vs HAVING

THANK YOU