What types of databases do you know?

Main types of databases:

• Relational databases (RDBMS): Store data in tables with clearly defined relationships. Examples: MySQL, PostgreSQL, Oracle.
• Document-oriented databases: Store data in document formats (JSON, BSON). Example: MongoDB.
• Key-value: Simple key-value stores, e.g., Redis, Riak.
• Columnar databases: Optimized for analytics, store data by columns. Example: Apache Cassandra, HBase.
• Graph databases: Store data as graphs (nodes and relationships). Example: Neo4j.
• Object-oriented databases: Store data as objects, close to programming objects.

The choice depends on the task: transactional systems more often use relational, for flexible schema — NoSQL databases.