Lesson 1: Introduction to Relational Databases

A relational database stores data in tables of rows and columns, related to each other by keys — the model behind Postgres, MySQL, and SQLite alike.

Real-time example

An e-commerce system has customers, orders, and products as separate tables, linked instead of duplicated everywhere.

CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100),
    email VARCHAR(100) UNIQUE
);
CREATE TABLE orders (
    id SERIAL PRIMARY KEY,
    customer_id INT REFERENCES customers(id),
    amount NUMERIC(10,2),
    created_at TIMESTAMP DEFAULT now()
);

What's happening

customer_id REFERENCES customers(id) is a foreign key — the database itself enforces that every order belongs to a real customer.

Model your real-world entities as tables first; the relationships between them (keys) come next.