query

A query is a request to a database to retrieve, insert, update, or delete data. In web and mobile development queries are usually SQL statements such as SELECT, INSERT, UPDATE and DELETE that return a result set or an execution status. Important considerations include how parameters are passed (e.g. parameterized queries), performance (indexes, joins, LIMIT, pagination) and security risks like SQL injection. MySQL is a client-server database with richer concurrency features; SQLite is an embedded database commonly used on devices with different constraints and optimizations.

Example: you build a mobile notes app that works offline using SQLite. To fetch the latest 20 notes for a user you use a parameterized query such as 'SELECT * FROM notes WHERE user_id = ? ORDER BY updated_at DESC LIMIT 20'. Using a parameter prevents SQL injection and lets you reuse the same query with different values. On a server-side API with MySQL you'd run similar queries but also create indexes on user_id and updated_at to speed things up, and you'd use prepared statements or an ORM to improve security and maintainability.