Databases

A database is a structured system for storing, managing, and retrieving information. They underpin nearly every modern application.

Key concepts

• Entity — a real-world object (e.g., a user, a product).
• Attribute — a property of an entity (e.g., name, price).
• Primary key — the unique identifier of a row.
• Foreign key — a reference from one table to another’s primary key.
• Index — speeds up lookups.

Types

Relational (SQL)

Tables with structured relations, queried via SQL.

CREATE TABLE Users (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    email VARCHAR(100) UNIQUE
);

See SQL.

Non-relational (NoSQL)

Flexible formats — documents, key-value, columnar, graph — designed for unstructured data and horizontal scaling.

{ "id": 1, "name": "Juan", "email": "juan@example.com" }

See NoSQL.

References

• W3Schools SQL
• MongoDB Documentation