Structured Query Language

What Is Structured Query Language?

Structured Query Language (SQL) is a domain-specific programming language designed for storing, retrieving, and manipulating data held in a relational database management system (RDBMS). Unlike general-purpose programming languages, SQL expresses operations in terms of sets and relations, allowing a user or application to describe what data is needed rather than how to retrieve it, leaving the database engine to determine an efficient execution plan. This declarative character has made SQL the dominant standard interface for relational data management since its inception.

SQL originated at IBM in the early 1970s as a language called SEQUEL (Structured English Query Language), developed by Donald Chamberlin and Raymond Boyce as a practical interface to the relational model proposed by Edgar Codd. Oracle, then known as Relational Software, shipped the first commercial SQL RDBMS in 1979. By 1986 the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO) had formally standardized SQL, and subsequent revisions, SQL-92, SQL:1999, SQL:2003, and SQL:2016, have progressively extended the language to cover object-relational features, XML, JSON, and analytic window functions. The AWS overview of SQL and relational databases traces this lineage from SEQUEL to the current ANSI/ISO standard.

The Relational Database Model

SQL is inseparable from the relational model on which it operates. A relational database organizes data into tables, each consisting of named columns with defined data types and rows that represent individual records. Relationships between tables are expressed through shared key values rather than embedded pointers, enabling flexible querying across multiple tables through join operations. This tabular organization, formalized in Codd's 1970 paper "A Relational Model of Data for Large Shared Data Banks," provides the mathematical foundation that SQL's set-based syntax is designed to query. The IEEE Xplore paper on SQL standards for object-relational databases examines how SQL standards evolved to accommodate more complex data models while preserving backward compatibility with the core relational model.

Core SQL Command Categories

SQL commands fall into several functional categories, each addressing a different aspect of database interaction. Data Definition Language (DDL) commands, principally CREATE, ALTER, and DROP, define and modify the structure of database objects such as tables, indexes, and views. Data Manipulation Language (DML) commands, including INSERT, UPDATE, DELETE, and SELECT, create, modify, delete, and retrieve records. The SELECT statement is the most frequently used SQL construct, supporting filtering with WHERE clauses, grouping with GROUP BY, and sorting with ORDER BY. Data Control Language (DCL) commands, such as GRANT and REVOKE, manage access permissions, while Transaction Control Language (TCL) commands, including COMMIT and ROLLBACK, govern the atomicity and consistency of multi-step operations.

SQL Standards and Extensions

While the ANSI/ISO SQL standard defines a common core, every major RDBMS extends that core with proprietary features. PostgreSQL, MySQL, Microsoft SQL Server, IBM Db2, and Oracle Database each implement the standard differently and add vendor-specific functions for full-text search, geospatial data, procedural programming, and performance tuning. SQL:2023 added features for property graph querying, reflecting the language's adaptation to graph-structured data. The ISO SQL standard page documents the current normative specification and revision history. Query optimization, the process by which the database engine selects an efficient execution plan for a given SQL statement, is a major area of ongoing RDBMS research and engineering.

Applications

Structured Query Language has applications across a wide range of computing domains, including: