Tell me about choosing a database: relational vs non-relational — when to choose what?

Relational databases (SQL) are well-suited for systems with clearly structured data and complex relationships, where integrity and transactional properties are important (e.g., banking systems, ERP). They provide ACID properties and allow the use of powerful JOIN queries.

Non-relational databases (NoSQL) are better chosen when data is unstructured or semi-structured, high scalability and schema flexibility are required. For example, for storing large volumes of logs, social media data, caching, or when data structures change frequently.

The choice depends on the requirements:

• If strict consistency and complex relationships are needed — relational.
• If scalability, schema flexibility, and high write speed are important — NoSQL.

Example: for an online store with orders and customers, a relational database is usually used, while for storing user sessions or cache — NoSQL.