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SQL Summary
If you've worked through every lesson, here's what you can now do — and where to go next.
You can
- Read and write
SELECTqueries, including filtering, sorting, paging, and pattern matching. - Aggregate with
COUNT,SUM,AVG,MIN,MAXand group results. - Combine tables with every join shape — and pick the right one for the question.
- Insert, update, and delete safely, with dry-runs and transactions.
- Design a schema with the right types, primary and foreign keys, and constraints.
- Add indexes where they pay off, and avoid the ones that don't.
- Spot — and prevent — SQL injection.
- Read across MySQL, PostgreSQL, SQL Server, and SQLite without surprise.
Where to go next
| Direction | Why |
|---|---|
| Window functions | Running totals, ranks, lag/lead — analytics without sub-queries. |
| CTEs & recursive CTEs | Break complex queries into named steps; traverse hierarchies. |
Reading EXPLAIN | Tell the database why a query is slow — and fix the index. |
| Transactions & isolation | Concurrency, deadlocks, MVCC — the next layer down. |
| An ORM | Laravel Eloquent, Django ORM, ActiveRecord, Drizzle. SQL is what they generate. |
| Database internals | B-trees, page caches, WAL — how the engine makes your queries fast. |
One last habit
Every time you write a query in real work, ask three questions:
- What's the simplest version that returns the right answer?
- What indexes would make it fast?
- What happens if the data doubles in size?
Tip: SQL is the rare technology where the syntax you learn today will still be valid in 20 years. Time spent here compounds.
Example
Example
-- You've learned: SELECT, JOIN, aggregate, -- subqueries, design, indexes, transactions.Try it Yourself »
Exercise
Next-step topic that powers running totals and ranks.
functions
Six letters; the OVER (…) family.
Test yourself
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