How to choose between SQL and NoSQL

Explained as simply as possible… but not simpler.

SQL and NoSQL solve the same problem (storing data) with fundamentally different approaches:

• SQL: Structured data in tables with predefined schemas. Relationships between tables via JOINs.

• NoSQL: Flexible data storage (documents, key-value, graph, wide-column). No fixed schema required.

SQL Databases

Key characteristic: Structured, relational, ACID-compliant

Structure:

Examples: PostgreSQL, MySQL, Oracle

Properties:

• ACID Transactions - All-or-nothing operations (critical for payments, banking)

• Complex Queries - JOINs across multiple tables, aggregations, filtering

• Data Integrity - Schema enforces structure, reduces errors

• Mature Tooling - Decades of optimisation, monitoring, backup tools

• Scaling

  • Vertical: Adding CPU/RAM to one server is expensive

  • Horizontal: Difficult to shard writes across multiple servers

• Rigid Schema - Changes require migrations, can lock tables

NoSQL Databases

Key characteristic: Flexible, distributed, eventually consistent

Types:

Properties:

• Horizontal Scaling - Add more servers to handle load

• Flexible Schema - Store different structures without migrations

• High Performance - Optimised for specific access patterns

• Distributed by Design - Natural fit for global, multi-region systems

• Eventual Consistency - Writes take time to propagate across nodes

• Limited Querying - Complex queries handled in application code

• Less Data Integrity - Schema validation happens in your application

A decision framework for interviews

Consistency Trade-off

Strong Consistency (SQL):

Eventual Consistency (NoSQL):

Scaling Patterns

SQL Scaling:

• Vertical: Add CPU/RAM to main server (Read replicas for read-heavy loads)

• Horizontal: Sharding (complex) - Split data across servers by key

NoSQL Scaling:

• Horizontal: Add nodes to cluster (Automatic data distribution)

Common Use Cases

---

Like posts like this?

By subscribing, you get a breakdown like this one every week.

Free subscribers also get a little bonus:

🎁 The System Design Interview Preparation Cheat Sheet

If you're into visuals, paid subscribers unlock:

→ My Excalidraw system design template – so you have somewhere to start
→ My Excalidraw component library – used in the diagram of this issue

No pressure though. Your support helps me keep writing, and I appreciate it more than you know ❤️