What is the difference between SQL and NoSQL databases?

SQL (relational) databases store data in structured tables with a predefined schema, use SQL for queries, enforce ACID transactions, and are excellent for complex queries with JOINs. Examples: PostgreSQL, MySQL, Oracle. NoSQL databases use flexible schemas (document, key-value, wide-column, graph models), are designed to scale horizontally, and prioritize availability and partition tolerance (BASE properties). Examples: MongoDB (document), Redis (key-value), Cassandra (wide-column), Neo4j (graph). Choose SQL for: complex relationships, strong consistency, and structured data. Choose NoSQL for: huge scale, flexible schema, or specialized data models.