SQL & Databases
SQL is the lingua franca of data engineering — it drives warehouse transformations, data checks, and analytics.
Learn it
Start with interactive, in-browser SQL so every concept ends in a query you run. Move from single-table SELECTs to JOINs, GROUP BY aggregation, and CTEs. Then study how relational databases store rows and columns and why schema and indexes matter for performance.
By the end you’ll be able to
- Write SELECT queries with filtering, sorting, and aggregation
- Join across tables and use subqueries/CTEs
Free resources
- Mode / ThoughtSpot — SQL Tutorial for Data Analysis(opens in a new tab)Start here
Run every query in-browser; strongest for analytics-style SQL.
Free~52 lessons - SQLBolt — Interactive SQL Lessons(opens in a new tab)
Fast, zero-setup drills for SELECT, JOINs, and DDL.
Free~18 lessons - Harvard CS50 — Introduction to Databases with SQL(opens in a new tab)
Deeper coverage of schema design and multiple engines.
Freeself-paced
Practice
Complete the Intermediate and Analytics sections of the SQL tutorial, writing JOIN + aggregation queries.
Build it — your output
A SQL query file answering 5 business questions using JOINs, aggregation, and at least one CTE.
Prove it — check yourself
You’re ready to move on when you can honestly say yes to each:
- Can you write a multi-table JOIN with a GROUP BY from scratch?
- Can you explain the difference between WHERE and HAVING?
- Can you describe when an index helps and what it costs?
Do it with AI
Use a free AI tool as your tutor and pair-programmer. Copy a prompt below into Claude, ChatGPT, or Gemini (for building, try Cursor or Copilot).
The habit: Try it yourself first — a little struggle is where the learning actually happens. Then ask for a hint before the answer — never copy blindly.
- Understand itClaude / ChatGPT / Gemini
I'm a beginner learning "SQL & Databases". Explain it in simple terms with a real-world analogy, then ask me ONE question to check my understanding — don't reveal the answer.
- PracticeClaude / ChatGPT / Gemini
Give me 3 short practice problems on "SQL & Databases" (my goal: write select queries with filtering, sorting, and aggregation), from easy to hard. Don't include the solutions yet — I'll try them first, then ask you to check.
- Get unstuckClaude / ChatGPT / Gemini
I'm stuck on this exercise about "SQL & Databases": [paste your attempt here]. Give me ONE small hint to move forward — do NOT give me the full solution.
- Build & reviewCursor / Copilot
Here is my work for a "SQL & Databases" task: [paste your code or answer]. Review it: point out what's wrong and WHY, and suggest improvements — but let me make the fixes myself.
- Prove itClaude / ChatGPT / Gemini
Quiz me on "SQL & Databases". Ask me 5 questions one at a time, wait for my answer each time, then tell me if I'm right and briefly why.