SELECT retrieves data, INSERT adds records, UPDATE modifies data, DELETE removes records. Use WHERE for filtering, ORDER BY for sorting, LIMIT for pagination.
INNER JOIN returns matching rows, LEFT JOIN includes all left table rows, RIGHT JOIN includes all right table rows, FULL JOIN combines both, CROSS JOIN creates cartesian product.
Document stores (MongoDB), Key-value (Redis), Column-family (Cassandra), Graph (Neo4j). NoSQL offers flexibility, scalability, and handles unstructured data well.
db.collection.find() queries documents, insertOne/Many adds data, updateOne/Many modifies documents, deleteOne/Many removes data. Use aggregation pipeline for complex queries.
SQL: structured data, ACID compliance, complex queries, vertical scaling. NoSQL: flexible schema, horizontal scaling, high performance, eventual consistency. Choose based on use case.