SQL · Lesson 1 of 8

SQL introduction

Databases, tables, rows and columns — and your first SELECT.

A relational database stores data in tables. Each table has columns (fields such as title, city) and rows (one record each). SQL — Structured Query Language — is how you ask the database questions and change its data.

The same core SQL works in MySQL, PostgreSQL, SQL Server, Oracle and SQLite. Our editor uses SQLite and a small fictional database with two tables:

companies — id, name, park, employees
jobs — id, title, company_id, city, min_exp, salary_lpa, posted_on

SELECT * FROM jobs; means “give me every column of every row in jobs”. Keywords are not case-sensitive, but writing them in capitals keeps queries readable. End each statement with ;.

Try it yourself

Code editor

Press Run to see the results.

Runs on a small fictional sample database (companies, jobs) that is rebuilt on every run, so you can't break anything.

Key points

Tables have columns (fields) and rows (records).
SELECT reads data; * means all columns.
End statements with a semicolon.

Quick check

In a table, what is one record called?

A column A row A schema A key

Practice task

Show every row from the companies table.

Interview corner

What is the difference between SQL and MySQL?

SQL is the query language. MySQL is one database system (an RDBMS) that understands SQL — others include PostgreSQL, SQL Server and Oracle.

What is a primary key?

A column (or set of columns) whose value uniquely identifies each row and can't be NULL — for example jobs.id.