What is a relational database and how does it differ from a non-relational one?

A relational database (RDB) is a type of database where data is organized into tables (relations) with clearly defined columns and rows. Each table has a schema that defines data types and relationships with other tables through keys (primary and foreign).

Main characteristics of relational databases:

• Strict data schema.
• Use of SQL for queries.
• Support for transactions and data integrity.
• Relationships between tables via keys.

A non-relational database (NoSQL) is a broader class of databases that do not use a tabular model. They can store data as documents (JSON, BSON), key-value pairs, graphs, or columns.

Differences:

• Flexible or absent schema (schema-less).
• Different data models (documents, graphs, columns).
• Often better horizontally scalable.
• May sacrifice strict integrity for performance and scalability.

Examples:

• Relational: PostgreSQL, MySQL
• Non-relational: MongoDB (documents), Redis (key-value), Neo4j (graphs)

The choice depends on data requirements, need for complex relationships, scalability, and consistency.