Structured Query Language — commonly known as SQL — is a language that is used to define, control, manipulate, and query data held in a relational database. SQL has been widely adopted since it was first developed in the early 1970s, and today it’s the predominant language used to interact with relational database management systems. SQL is ideal for managing structured data, which is data that can fit neatly into an existing data model. Learning how to use SQL is an essential skill for developers and system administrators in a wide variety of contexts.
The SQL standard is defined by two organizations: the American National Standards Institution (ANSI) and the International Organization for Standardization (ISO). However, relational database management systems typically implement their own custom subset of SQL, and these only rarely come close to full compliance with the SQL standard. Often these unique implementations include special features — known as SQL extensions — which aren’t included in the standard, but provide users with useful functionality that may not be available in other SQL implementations.
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