SQL Introduction

SQL (Structured Query Language) is the standard language for storing, reading and changing data in relational databases. Banks, shops, hospitals, apps and test frameworks all keep their data in databases, and SQL is how you talk to them. This lesson explains what SQL is, how a database is organised and what kinds of commands SQL has, then you run your first queries.

What is SQL?

SQL stands for Structured Query Language. It is pronounced “S-Q-L” or “sequel”. You use it to:

• read data: which customers ordered this month?
• add, change and delete data: record a new order, update a price, remove a test account.
• define the structure: create tables and the rules the data must follow.
• control access: decide who may read or change what.

SQL is declarative: you describe what result you want, and the database works out how to get it. You never write loops to search a table yourself.

Databases, tables, rows and columns

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

• each column has a name and a data type, for example salary INT;
• each row is one record, for example one department;
• a primary key column holds a value that identifies each row uniquely, such as dept_id.

Here is the departments table from the sample database used throughout this tutorial:

Tables are linked by keys. Every employee row stores a dept_id, which points at a row in departments. That link is called a foreign key, and it is what makes the database relational.

What is an RDBMS?

An RDBMS (Relational Database Management System) is the software that stores the tables and runs your SQL. Popular ones:

SQL vs MySQL

SQL is the language; MySQL is a database system that understands it. The core of SQL (SELECT, INSERT, JOIN, GROUP BY …) is an ISO standard and works almost the same everywhere. Each product adds its own extras. This tutorial uses MySQL 8.4 LTS, and every result you see was produced by a real MySQL 8.4 server. Where MySQL differs from other databases, the lesson says so.

Your first SQL queries

Ask the database which departments exist and where they are:

SELECT dept_name, locationFROM departments;

Now a question with a condition and a sort: who earns more than 90,000, highest first?

SELECT first_name, job_title, salaryFROM employeesWHERE salary > 90000ORDER BY salary DESC;

The five types of SQL commands

You will learn each group in this tutorial: queries first, because reading data is what you will do most.

Why learn SQL?

• It is one of the most requested skills for developers, testers, data analysts and data engineers.
• It has barely changed in decades, so what you learn now stays useful.
• Testers use SQL to set up test data and check what an application really saved, not just what the screen shows.
• Python, Java and every other language talk to databases by sending SQL.

SQL introduction FAQ