Tutorials System Design Tutorial
SQL Databases in Large Systems — Complete Guide
SQL Databases in Large Systems — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of System Design Tutorial on Toolliyo Academy.
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System Design Tutorial · Lesson 21 of 100
SQL Databases in Large Systems
Basics → Scale → Interview
Basics · 1 — Building blocks · ~6 min · Module 3: Database Systems
What is this?
Relational SQL databases give strong schemas, joins, and ACID transactions — still the default for ShopNest orders and payments.
Why should you care?
Money and stock need constraints and multi-row transactions that document stores do not give you for free.
See it live — copy this example
Sketch the architecture on paper. These lessons focus on concepts and trade-offs.
ShopNest OLTP (Postgres/SQL Server):
Orders 1—* OrderItems
BEGIN; insert order; decrement stock; COMMIT;
Scale path: indexes → read replicas → partitioning → shard only when neededRun Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Start with a solid relational model and indexes.
- Add replicas for reads.
- Shard only after vertical and caching options are exhausted.
Practice next
- Model Orders and OrderItems with FKs.
- Put checkout writes in one transaction.
- Add a read replica for reporting.
- Move heavy reports to a replica or warehouse.
- Add idempotency_key UNIQUE on orders.
Remember
SQL shines for transactional cores. Scale reads with replicas. Shard last, not first.
ShopNest order DB
Checkout uses relational ACID transactions.
Outcome: Stock and order rows stay consistent under retries.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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