What is a database?

A database is an organized collection of information that can be stored and queried. A database management system is the software managing it. Databases help applications reliably handle product catalogues, reservations, accounts and inventory.

What do tables, queries, indexes and transactions do?

Relational databases organize rows and columns into tables that can be linked using keys. SQL is a language for querying and changing data in such systems. Indexes can speed particular searches while consuming storage and adding update costs. Transactions coordinate related steps so that they complete consistently: reducing stock and recording an order, for example, should not become disconnected. Integrity constraints help limit invalid or duplicate records. NoSQL is a broad group of different data models rather than one structure.

How are reliability and security maintained?

A model should reflect the relationships and queries required, rather than assuming the largest system suits every task. Access should match responsibilities, and inputs require safe query techniques. Backups, replication and transaction logs serve different purposes. A copy on another server is not equivalent to a versioned backup that can survive accidental deletions propagating. Seeing successful backup jobs is insufficient without restoration tests. Freshness, consistency and the retention period for personal data are also parts of responsible database management.