Query

A query is a request for information from a data source or information system. It can be issued as a text string or constructed programmatically and is used in contexts such as databases, search engines, and web services. The term originates from the Latin quaerere, meaning “to seek.”

Queries fall into several broad categories. Database queries retrieve data from structured stores and are often written in query languages such as SQL for relational databases or specialized languages for NoSQL systems. Search queries are formulated to obtain information from indexing systems or search engines and may use keywords, natural language, or advanced operators. Graph and semantic data queries use languages such as SPARQL to express patterns over interconnected data. API queries typically pass parameters in a request URL or payload to obtain resources or perform actions. Related concepts include query-by-example and structured query language constructs that specify projections, filters, and ordering.

Processing a query involves parsing and validating the input, optimizing the query plan, and executing it against the data store. Optimizers select efficient access paths, often leveraging indexes, statistics, and caching. Result sets are returned to the caller and may support features such as pagination, sorting, and aggregation. Performance depends on data size, index design, and system load, while security concerns include input validation to prevent injection attacks and enforcement of access controls.