Answer a business question with SQL

SELECT basics — filter, sort, limit

🗄️ SQL Essentials · The bread and butter: SELECT columns FROM table WHERE conditions ORDER BY sort LIMIT n. Learn this well; everything else builds on it.

Joins — INNER, LEFT, RIGHT, FULL

🗄️ SQL Essentials · Combine rows across tables using keys. Pick the right join type or you'll silently lose or duplicate data.

GROUP BY & aggregates

🗄️ SQL Essentials · Roll up rows into buckets and summarize: COUNT, SUM, AVG, MIN, MAX. The foundation of every report.

Subqueries & CTEs

🗄️ SQL Essentials · Nest one query inside another, or pull it out as a named CTE (WITH clause) for readability. The tool that turns 200-line queries into 20-line ones.

Window functions — ranking, running totals, LAG/LEAD

🗄️ SQL Essentials · Aggregate without collapsing rows. Ranks, running totals, period-over-period comparisons — the superpower of modern SQL.

Query optimization — anti-patterns to avoid

🗄️ SQL Essentials · The top 10 mistakes that turn fast queries slow: SELECT *, function on indexed column, N+1, implicit casts, deep pagination.