SQL Tutorial

Query relational data — SELECT, JOIN, GROUP BY, and indexes, with runnable examples.

1 min read 162 words Updated Mar 2, 2026

SQL is the language of relational databases. Learn to read and shape data with a few core statements.

Selecting rows

SELECT id, name, price
FROM products
WHERE price > 10
ORDER BY price DESC
LIMIT 10;

Filtering and patterns

SELECT * FROM users
WHERE email LIKE '%@example.com'
  AND created_at >= '2026-01-01';

Aggregating with GROUP BY

SELECT category, COUNT(*) AS n, AVG(price) AS avg_price
FROM products
GROUP BY category
HAVING COUNT(*) > 5;

Joining tables

SELECT o.id, u.name, o.total
FROM orders o
JOIN users u ON u.id = o.user_id
WHERE o.status = 'paid';

Join type	Returns
`INNER`	rows with matches in both tables
`LEFT`	all left rows, nulls on no match
`FULL`	all rows from both tables

Creating tables and indexes

CREATE TABLE products (
  id        SERIAL PRIMARY KEY,
  name      TEXT NOT NULL,
  price     NUMERIC(10, 2),
  category  TEXT
);

CREATE INDEX idx_products_category ON products (category);

Indexes speed up reads but slow down writes. Add them on columns you filter or join on often.