SQL Introduction

What is SQL?

SQL (Structured Query Language, pronounced "S-Q-L" or "sequel") is the standard language for working with relational databases — PostgreSQL, MySQL, SQL Server, Oracle, SQLite and many more. With SQL you can:

• query data (SELECT),
• add, change and remove data (INSERT, UPDATE, DELETE),
• define tables and their rules (CREATE TABLE),
• control access and transactions.

Tables, rows and columns

A relational database stores data in tables. Each row is one record (one customer, one order) and each column is one attribute (name, email, price). Tables are linked through keys — for example, every order stores the customer_id of the customer who placed it.

Your first query

SELECT name, city FROM customers;

name | city
Ada Lovelace | London
Alan Turing | Manchester
Grace Hopper | New York
Linus Torvalds | Portland
Margaret Hamilton | Boston
Tim Berners-Lee | Geneva
Barbara Liskov | NULL
Guido van Rossum | Amsterdam
Yukihiro Matsumoto | Tokyo
Katherine Johnson | Hampton

Running SQL on this site

Every example runs on a real SQLite database inside your browser, loaded with a small shop and company dataset — see The Sample Database. Each run starts from a fresh copy, so feel free to INSERT, UPDATE and DELETE. The SQL you learn here works almost identically in PostgreSQL and MySQL; differences are pointed out where they matter.