SQL Introduction

Every app that remembers anything (your orders, messages, followers, progress through a course) stores that data in a database. SQL (Structured Query Language, often pronounced “sequel”) is the language used to talk to most databases. It’s one of the most useful and long-lasting skills in tech: developers, data analysts, product managers and marketers all use it.

What is a database?#

A database is an organised collection of data that a program can search and update quickly and safely. The most common kind is a relational database, which stores data in tables, like spreadsheets:

id	name	city
1	Asha	Pune
2	Rohan	Delhi
3	Meera	Jaipur

• Each row is one record (one customer).
• Each column is one piece of information (name, city).
• Tables can be related to each other: an orders table can point to the customer who placed each order. That’s what “relational” means.

Why not just use a spreadsheet?#

Databases are built for things spreadsheets struggle with:

• Size: millions or billions of rows, still fast.
• Many users at once: thousands of people placing orders at the same moment without overwriting each other.
• Rules: every order must belong to a real customer; an email must be unique.
• Safety: changes are saved reliably even if the server crashes mid-way.
• Questions in code: “total sales per city last month” is one SQL statement.

Your first SQL query#

SELECT name, city
FROM customers
WHERE city = 'Pune';

Read it like English: “Select the name and city from the customers table where the city is Pune.” The database answers with a small table of results:

name	city
Asha	Pune
Kabir	Pune

SQL is declarative: you describe what you want, and the database works out how to get it fast.

What SQL can do#

SQL statements fall into a few groups:

• Query data: SELECT (most of what you’ll write).
• Change data: INSERT, UPDATE, DELETE.
• Define structure: CREATE TABLE, ALTER TABLE, DROP TABLE.
• Control access: GRANT, REVOKE (for administrators).

This course covers the first three groups.

Popular database systems#

Database	Where you’ll meet it
MySQL / MariaDB	WordPress and most shared hosting (including this site)
PostgreSQL	Modern web apps and startups; very feature-rich
SQLite	Inside phones, browsers and apps; a database in one file
SQL Server	Large companies using Microsoft tools
Oracle	Banks and big enterprises

They all speak SQL. The core you’ll learn here (SELECT, WHERE, JOIN, GROUP BY…) works the same in all of them; only small details differ, and we’ll point them out.

SQL syntax basics#

-- This is a comment
SELECT name FROM customers;   /* so is this */

select NAME from Customers;   -- keywords aren't case-sensitive

• Statements end with a semicolon.
• SQL keywords are not case-sensitive, but writing them in UPPER CASE is the common style and makes queries easier to read.
• Text values go in single quotes: 'Pune'. (Double quotes are for column or table names in standard SQL.)
• Spaces and new lines don’t matter; spread long queries over several lines.
• Whether table names and text comparisons are case-sensitive depends on the database system and its settings.

How to practise (free, nothing to install)#

You need a database to run queries. The easiest options:

1. sqliteonline.com or DB Fiddle (db-fiddle.com): run SQL in your browser. Choose SQLite, MySQL or PostgreSQL.
2. DB Browser for SQLite: a free desktop app; the whole database is one file.
3. Later, install MySQL or PostgreSQL locally, or use the database from your hosting panel (phpMyAdmin).

In the next lesson, you’ll create the practice database used throughout this course with one copy-and-paste script.

Where SQL is used#

• Web development: every login, comment and product page reads from a database.
• Data analysis: answering business questions from sales, traffic or app data.
• Reporting tools like Looker Studio, Power BI and Metabase are built on SQL.
• AI and data science: collecting and cleaning the data that models learn from.

Common mistakes#

• Double quotes around text values: use 'Pune', not "Pune" (MySQL allows both, PostgreSQL doesn’t).
• Forgetting the semicolon when running several statements.
• Expecting rows in a particular order without asking for it (you’ll learn ORDER BY).

Exercise#

1. Open sqliteonline.com (or any tool above) and run SELECT 1 + 1; to check it works.
2. Run SELECT 'Hello, SQL!' AS greeting;.
3. Think of an app you use daily. Write down three tables it probably has and three columns in each.

Summary#

• Relational databases store data in related tables of rows and columns.
• SQL is the language for querying and changing that data; you say what you want, not how.
• MySQL, PostgreSQL and SQLite all share the same core SQL.
• Practise for free in the browser with sqliteonline.com or DB Fiddle.

Next, you’ll create tables and load the practice database for this course.