SQL Tutorial for Beginners: Real Queries for Real Jobs

01.Our example schema

Imagine an online store with three tables. We'll use this throughout.

customers(id, name, city)
orders(id, customer_id, total, created_at)
order_items(id, order_id, product, qty, price)

02.SELECT, WHERE, ORDER BY

The three clauses cover the majority of analyst queries. Always reach for `LIMIT` when you're exploring a new table — production databases punish people who forget.

SELECT name, city
FROM customers
WHERE city = 'Chennai'
ORDER BY name
LIMIT 50;

03.JOINs without the confusion

INNER JOIN keeps only matches. LEFT JOIN keeps everything from the left table. RIGHT JOIN is rarely used in practice — flip the tables and use LEFT instead.

SELECT c.name, COUNT(o.id) AS order_count
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.id
GROUP BY c.name
ORDER BY order_count DESC;

04.GROUP BY and aggregates

SUM, COUNT, AVG, MIN, MAX. Always include every non-aggregated column in GROUP BY. PostgreSQL will refuse to run the query otherwise — that's a feature, not a bug.

05.Window functions for ranking

Window functions compute a value across a set of rows without collapsing them. Useful for ranking, running totals and de-duplication.

SELECT id, customer_id, total,
       ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY created_at DESC) AS recency
FROM orders;

06.EXPLAIN: read the query plan

When a query is slow, prefix it with `EXPLAIN ANALYZE`. Look for `Seq Scan` on big tables (usually bad) and missing indexes. Adding the right index is often a 100x speedup.