SQL databases

Beginner 12 min read

Tables, keys and queries: how applications store data reliably in a relational database.

Why it matters

Almost every application keeps its important data in a SQL database. A good table design and a few correct queries prevent slow pages, duplicated data and lost updates.

Where it fits

What you will learn

Create a table with a primary key and the right column types.
Insert, select, filter and sort rows.
Explain what a transaction guarantees.

Before you start

HTTP and APIs✓ Learned

Tables, rows and keys

A table is a list of rows with the same columns. A primary key identifies each row; a foreign key points to a row in another table. The database enforces these rules, so bad data cannot get in.

Example

CREATE TABLE users (
    id INTEGER PRIMARY KEY,
    email TEXT NOT NULL UNIQUE
);

INSERT INTO users (email) VALUES ('ana@example.com');

SELECT id, email FROM users ORDER BY email;

Transactions

A transaction groups statements so that they all succeed or none do — moving money between two accounts must never apply only half. Use them whenever one action changes several rows.

Watch out: Build queries with parameters (placeholders), never by joining user input into the SQL string. That is how SQL injection happens.

Install and set up

PostgreSQL downloads ↗ Installers and packages for every operating system.

Official documentation and references

Documentation

SQLite documentation ↗ Official · SQLite — The small file-based database Zerethon IDE runs SQL examples with.

Guides

PostgreSQL tutorial ↗ Official · PostgreSQL

Practise in Zerethon IDE

Code Help: SQL — CREATE TABLE and INSERT Copyable examples with their output.
Code Help: SQL — Transactions
Algorithm sample: SQL Basics · read the code, then use Open in IDE on its page A runnable SQL sample you can open in the IDE.

Learn next

Git basics ›✓ Learned

Schema migrations✓ Learned
Backups and recovery✓ Learned
HTTP and APIs✓ Learned

Done with this topic?

Reviewed 2026-10-23. Links go to the official documentation unless marked otherwise.