Introduction to SQL

What Is SQL?

SQL is a language for working with data stored in tables inside a relational database (MySQL, PostgreSQL, SQLite, SQL Server, and others all use SQL with minor dialect differences). Unlike R or Python, SQL is declarative - you describe WHAT data you want, and the database figures out HOW to get it.

You do not need to install anything heavy to practice: SQLite is a free, file-based database built into Python, and free browser tools like DB Fiddle or SQLite Online let you run queries instantly.

Tables, Rows, and Columns

A database is a collection of tables. Each table is a grid: columns define the type of information (name, age, score), and rows are individual records. This guide uses a simple students table throughout its examples.

-- the students table used throughout this guide
-- id | name    | age | score | class
-- 1  | Rafi    | 23  | 88    | A
-- 2  | Meera   | 25  | 92    | A
-- 3  | Zayn    | 21  | 76    | B
-- 4  | Nadia   | 22  | 65    | B

Your First Query

SELECT * FROM students;

id | name  | age | score | class
1  | Rafi  | 23  | 88    | A
2  | Meera | 25  | 92    | A
3  | Zayn  | 21  | 76    | B
4  | Nadia | 22  | 65    | B

SELECT * means "give me every column"; FROM students tells the database which table to read from. The semicolon marks the end of the statement.

SQL Keywords Are Not Case-Sensitive (But Style Matters)

-- these two queries do exactly the same thing
select * from students;
SELECT * FROM students;

Comments

-- a single-line comment starts with two dashes
SELECT name FROM students; -- inline comments work too

/* a multi-line comment
   spans several lines like this */