Structured Query Language (SQL)

SQL (Structured Query Language) is a domain-specific language used for managing and querying data held in relational [database](/en/terms/relational-database) management systems (RDBMS). It is a declarative language, meaning users specify what data they want, and the database engine determines how to retrieve or manipulate it. The core components of SQL include Data Definition Language (DDL) for defining database schemas (e.g., CREATE TABLE, ALTER TABLE), Data Manipulation Language (DML) for inserting, updating, and deleting data (e.g., INSERT, UPDATE, DELETE), and Data Control Language (DCL) for managing permissions (e.g., GRANT, REVOKE). The most commonly used part is the Data Query Language (DQL), primarily the SELECT statement, which allows complex data retrieval through filtering (WHERE), sorting (ORDER BY), aggregation (GROUP BY), and joining multiple tables based on defined relationships. SQL databases store data in tables consisting of rows (records) and columns (attributes), enforcing data integrity through constraints like primary keys, foreign keys, and unique constraints. Standardization efforts (e.g., by ANSI/ISO) ensure a degree of interoperability, although specific RDBMS vendors (like Oracle, MySQL, PostgreSQL, SQL Server) implement proprietary extensions and variations.