SQL Exercises

Introduction

Theory only sticks once you've solved real problems. This lesson is a curated set of beginner-to-advanced exercises you should run on a real DB.

Understanding the topic

Core concepts to understand:

• Top-N customers by spend.
• Find duplicates in any table.
• Daily new sign-ups for last 30 days.
• Most-bought products per category.
• Users who haven't bought in 90 days.

Syntax reference

Visual workflow / architecture:

Pattern bank
 1. Top-N per group       → window functions
 2. Duplicates           → GROUP BY + HAVING
 3. Time series           → date_trunc + GROUP BY
 4. Funnel                → CASE + SUM
 5. Inactive users        → LEFT JOIN + IS NULL

Real-world use

Every interview at every product/data company tests these patterns. Solving 20 of them fluently puts you ahead of 90% of candidates.

Best practices

• Solve on a real Postgres/MySQL — not paper.
• Add EXPLAIN ANALYZE for each.
• Time yourself.

Hands-on exercise

Interview preparation — practice these questions:

• Q1. Top-3 spenders per country.
• Q2. Find duplicate emails.
• Q3. New sign-ups per day for 30 days.
• Q4. Funnel: signed up → activated → paid.
• Q5. Users inactive for 90 days.