Introduction to SQL

SQL (Structured Query Language, often said "sequel") is the language for asking questions of a relational database. Banks, shops, hospitals, apps and websites keep their data in databases, and SQL is how they read and change it.

Tables, rows and columns

A relational database stores data in tables. Each table looks like a spreadsheet:

• each column holds one kind of information (a name, a price, a date),
• each row is one record (one customer, one product, one order).

One language, many databases

SQL is a standard, so what you learn here works, with small differences, in every major database:

Database	Where you'll meet it
SQLite	inside phones, browsers and apps; this course's examples run on it
MySQL	websites and web apps
PostgreSQL	apps and analytics; it follows the standard closely
SQL Server	companies built on Microsoft tools, and Azure SQL Database in the cloud
Oracle	large enterprises such as banks

Your practice database

Every SQL example on CodeMIRAI runs against a small online shop database, right in your browser. It has five tables:

Run this query to see every customer:

SELECT * FROM customers;

SELECT * means "all columns" and FROM customers says which table. The semicolon ends the statement.

Asking a more specific question

SQL reads almost like a sentence. This asks for the name and price of every product that costs under 50, cheapest first:

SELECT name, price
FROM products
WHERE price < 50
ORDER BY price;

Press Edit, change 50 to 100, and run it again.

SQL keywords

Words like SELECT, FROM and WHERE are keywords. SQL does not care whether you write them in capitals, but writing them in capitals is a strong convention because it makes queries easier to read.