Relational Databases

Relational databases organize data into tables with rows and columns, linked by keys.

Syntax

COMPUTER-SCIENCE

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

CREATE TABLE orders (
    id INT PRIMARY KEY,
    user_id INT,
    FOREIGN KEY (user_id) REFERENCES users(id)
);

SQL JOIN Types

SQL

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-- INNER JOIN
SELECT users.name, orders.amount
FROM users
INNER JOIN orders ON users.id = orders.user_id;

-- LEFT JOIN (all users, even without orders)
SELECT users.name, COALESCE(SUM(orders.amount), 0) AS total
FROM users
LEFT JOIN orders ON users.id = orders.user_id
GROUP BY users.name;

-- Subquery
SELECT name FROM users
WHERE id IN (SELECT user_id FROM orders WHERE amount > 100);

Try It Yourself

Practice

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Exercise

What is a foreign key?

Answer

A column in one table that references the primary key of another table, creating a relationship.

Quick Quiz

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What does SQL stand for?

Simple Query Language

Structured Query Language

Standard Query Logic

System Query Language

SQL stands for Structured Query Language.

Retry Quiz

Interview Questions

What are ACID properties?

Atomicity, Consistency, Isolation, Durability — properties that guarantee reliable database transactions.

Next Lesson

NoSQL Databases