SQL - Wikipedia
https://en.wikipedia.org/wiki/SQL
SQL is a domain-specific language used in programming and designed for managing data held in a relational database management system (RDBMS), or for stream processing in a relational data stream management system (RDSMS).
SQL Tutorial
https://www.w3schools.com/sql/default.asp
SQL is a standard language for storing, manipulating and retrieving data in databases. Our SQL tutorial will teach you how to use SQL in: MySQL, SQL Server, MS Access, Oracle, Sybase, Informix...
Справочник mySQL | Трепачёв Дмитрий
http://old.code.mu/sql/
Учебник PHP+SQL для новичков. Фреймворки JavaScript. Задачник PHP+SQL с нуля. Задачник по фреймворкам JavaScript.