What is the difference between relational and non-relational databases? What types of databases have you worked with?

Relational databases (RDBMS) store data in tables with well-defined schemas and relationships between tables through keys. They use SQL for queries and are well-suited for structured data with transactional requirements.

Non-relational databases (NoSQL) can store data in various formats: documents, key-value, graphs, columns. They are more flexible in terms of schema, scale horizontally, and are suitable for large volumes of data with less strict structural requirements.

Main differences:

• Schema: fixed (relational) vs flexible (non-relational)
• Query language: SQL vs various (e.g., JSON queries, proprietary APIs)
• Scalability: vertical (relational) vs horizontal (non-relational)

Types I have worked with:

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

Example: for complex transactions and reporting, relational databases are better; for session storage or caching, Redis.