Introduction to Databases

Modern applications depend on data.

When you use a banking app, shopping site, school portal, or social platform, data is constantly being stored and retrieved.

Examples:

user accounts
passwords
products
payments
messages
bookings
reports

That data must stay available even after the program closes.

Databases are designed for this purpose.

What Is a Database?

A database is an organized system used to store, manage, and retrieve data efficiently.

Instead of keeping information in random files, a database stores data in a structured way.

A good database system allows you to:

save data permanently
search quickly
update safely
delete records cleanly
organize relationships
handle large amounts of data

Why Not Just Use Lists or Text Files?

Python lists, dictionaries, and text files are useful for beginner projects.

Example:

python

users = [
    {"name": "Tom", "age": 25},
    {"name": "Maya", "age": 30}
]

This works for small programs.

But as data grows, problems appear:

hard to search large data
no built-in relationships
duplicate data issues
poor performance
harder updates
difficult multi-user access
weak security

Databases solve these problems.

What Is Structured Data?

Structured data means data organized into clear fields.

Example user data:

id
name
email
age

Databases store this in tables.

Tables, Rows, and Columns

A table is like a spreadsheet.

Example:

text

users
+----+------+------------------+-----+
| id | name | email            | age |
+----+------+------------------+-----+
| 1  | Tom  | tom@mail.com     | 25  |
| 2  | Maya | maya@mail.com    | 30  |
+----+------+------------------+-----+

Columns

Columns describe what kind of data is stored.

Examples:

id
name
email
age

Rows

Each row is one complete record.

Example:

Tom, tom@mail.com, 25

What Is SQL?

SQL stands for Structured Query Language.

It is the language used to communicate with relational databases.

With SQL, you can:

create tables
insert data
read data
update data
delete data
filter results
join tables
manage permissions

SQL is one of the most important skills in software development.

Example SQL Query

Get all users:

sql

SELECT * FROM users;

Add a user:

sql

INSERT INTO users (name, age)
VALUES ('Tom', 25);

Update a user:

sql

UPDATE users
SET age = 26
WHERE name = 'Tom';

Delete a user:

sql

DELETE FROM users
WHERE name = 'Tom';

What Is CRUD?

CRUD represents the four most common database actions.

Create

Add new data.

sql

INSERT INTO users ...

Read

View data.

sql

SELECT * FROM users;

Update

Change existing data.

sql

UPDATE users ...

Delete

Remove data.

sql

DELETE FROM users ...

Most apps are built around CRUD operations.

What Is a Relational Database?

A relational database stores data in tables that can be related to each other.

Example:

users table

text

id | name
1  | Tom
2  | Maya

orders table

text

id | user_id | total
1  | 1       | 50
2  | 2       | 80

The user_id links orders to users.

This relationship is why it is called a relational database.

What Is an RDBMS?

RDBMS stands for Relational Database Management System.

It is the software that manages relational databases.

Examples:

SQLite
PostgreSQL
MySQL
Oracle
Microsoft SQL Server

The database is the data itself.

The RDBMS is the software used to manage that data.

Think of it like:

Data = books
RDBMS = library system

Popular RDBMS Tools

SQLite

lightweight
file-based
great for beginners
built into Python

PostgreSQL

powerful
advanced features
excellent for production apps

MySQL

popular
widely used in web development
strong community support

What Is DDL?

DDL stands for Data Definition Language.

DDL commands define or change database structure.

You use DDL to create or modify tables.

Common DDL commands:

CREATE
ALTER
DROP
TRUNCATE

Examples

Create table:

sql

CREATE TABLE users (
    id INTEGER,
    name TEXT,
    age INTEGER
);

Add a column:

sql

ALTER TABLE users
ADD COLUMN email TEXT;

Delete table:

sql

DROP TABLE users;

What Is DML?

DML stands for Data Manipulation Language.

DML commands work with the data inside tables.

Common DML commands:

INSERT
UPDATE
DELETE

Examples

Insert:

sql

INSERT INTO users (name, age)
VALUES ('Tom', 25);

Update:

sql

UPDATE users
SET age = 26
WHERE name = 'Tom';

Delete:

sql

DELETE FROM users
WHERE name = 'Tom';

What About SELECT?

SELECT is often placed in a separate category called DQL.

DQL = Data Query Language.

Used to read data.

sql

SELECT * FROM users;

Some beginners group it under DML, but knowing DQL is helpful.

What Is a Primary Key?

A primary key is a column that uniquely identifies each row.

Example:

text

id

No two rows should have the same primary key.

This helps databases track records safely.

Why Databases Matter in Python

Python programs often need real storage.

Examples:

login systems
inventory systems
finance trackers
student portals
booking apps
dashboards

Without databases, large apps become difficult to manage.

Code Along

For a school system, write 3 tables you would create.

Example:

students
courses
enrollments

Then write two columns for each.

Mini Challenge

An online store needs:

users
products
orders

Tasks:

Write one table for each
Choose a primary key
Write one relationship between tables
Write one SQL SELECT query example

Example answer:

text

products(id, name, price)

Real World Use Case

Every major platform uses databases:

banks
hospitals
schools
e-commerce
social media
government systems

Quiz

What does SQL stand for?
What is the difference between a database and an RDBMS?
What does CRUD mean?
Name two DDL commands.
Name two DML commands.
What is a primary key?
Why are relational databases called relational?

Assignment

Choose one app idea and write:

3 tables
columns for each table
one primary key per table
two CRUD actions users would perform

Summary

You learned the foundations of databases, including SQL, CRUD, tables, relational databases, RDBMS, DDL, DML, and why databases are essential in real Python applications.