Can you explain how non-relational databases differ from relational ones and their main features?

Relational databases (RDB) and non-relational databases (NoSQL) differ in data storage structure and organization principles.

Relational databases:

• Store data in tables with clearly defined schemas (rows and columns).
• Use SQL for queries.
• Ensure data integrity through constraints and transactions.
• Suitable for complex relationships between data.

Non-relational databases:

• Store data in various formats: documents (JSON, BSON), key-value, graphs, column stores.
• Do not require a fixed schema, providing flexibility in data structure.
• Often provide horizontal scaling and high performance.
• May sacrifice some consistency for speed (CAP theorem).

Example:

• Relational DB: PostgreSQL, MySQL
• Non-relational DB: MongoDB (document), Redis (key-value), Neo4j (graph)

The choice depends on data structure requirements, scalability, and operation types.