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Lessons (21)

• SQL Language, Identifiers & Data Types

  Interview answers for DDL/DML/DCL, generated IDs, UNION, static/dynamic SQL and CHAR/VARCHAR. Covers PDF questions …

• SQL Interview Query Drills

  … Covers PDF questions 23, 25 and 26. ![SQL drill workflow: define grouping, compute an aggregate, then filter with …

• SQL vs NoSQL Databases

  … tables, rows, relationships sql CREATE TABLE customers ( id BIGSERIAL PRIMARY KEY, email TEXT NOT NULL UNIQUE, name TEXT …

• Database Indexing & Transactions

  … by country, then city, then age. sql CREATE INDEX idx people location ON people (country, city, age); Query …

• Onion Architecture & Clean vs Hexagonal vs Onion

  … infrastructure, UI, tests Implementations of the interfaces, web framework, DB, external APIs SqlOrderRepository , controllers, EF Core / Prisma mermaid …

• Flash Sale: 10 Million Buyers, No Oversell

  … oversell still needs a durable owner and reconciliation. sql UPDATE inventory SET available = available - 1 WHERE sku = :sku …

• Storage Engines & Database Operations

  … the signals behind database reliability. What happens below SQL matters A storage engine decides how rows and indexes …

• N+1 Queries, Loading & Pagination

  … In production, sample traces rather than logging sensitive SQL values.

• Indexes, EXPLAIN & Query Plans

  … checklist Capture slow query fingerprints, not only raw SQL text. Compare p50 and p99. Refresh statistics after data …

• Layered (N-Tier) Architecture

  … No SQL in controllers; no HTTP in services. 2. Dependencies point down: a layer may use the layer …

• Authentication, Authorization & API Security

  … tokens MFA, rate limiting, short token lifetimes Injection (SQL, NoSQL, command) ' OR 1=1 -- returns every row Parameterised …

• Vertical Slice Architecture & the Modular Monolith

  … simple, direct SQL export async function getOrderHistory(db: Pool, customerId: string) { const { rows } = await db.query( SELECT id …

• Data in Microservices: Sagas & the Outbox Pattern

  … or CDC / Debezium)"] -- K[(Kafka / RabbitMQ)] -- C[Consumers] sql BEGIN; INSERT INTO orders (id, customer id, total cents …

• Clean Architecture in Depth

  … no req , no res , no SQL, no Stripe SDK, no JSON. Just the story of placing an order …

• S — Single Responsibility Principle

  … rules from ACCOUNTING / } reportHours() { / format from HR / } save() { / SQL from the DBA / ops team / } } mermaid flowchart TB A …

• DRY: Don't Repeat Yourself

  … between files One .env / config module The same SQL in many places A repository function Repeated UI markup …

• Clean Error Handling

  … to exit a loop). - Showing stack traces or SQL errors to end users. That is a security leak …

• Deployments, ReplicaSets & Rollouts

  … StatefulSets) is possible, but managed databases (RDS, Cloud SQL) are often simpler and safer. Decide deliberately. Key takeaways …

• Kafka vs RabbitMQ & the Kafka Ecosystem

  … Apache Flink (a powerful separate processing cluster), ksqlDB (SQL over streams). Schema Registry: contracts for events Stores versioned …

• What Is Software Architecture?

  … The team knows SQL well. Expected volume: 50k orders/day. Decision Use PostgreSQL 17 (managed, Amazon RDS). Consequences …

• Choosing an Architecture

  … fix boundaries Business rules scattered across controllers and SQL Need a real domain core (Clean / Hexagonal) Six layers …