The current century can without exaggeration be called the century of information, but information without the ability to sample and process it is nothing more than a set of bytes in data stores. This ability to efficiently sample and process data has provided developers with SQL for decades. It is closely related to relational databases. But despite this, recently, when large amounts of data are becoming increasingly important, the mentioned language is developing in such a way as to facilitate the search for data in databases built on various technologies. This book is distinguished by a wide coverage of topics (from SQL basics to such complex issues as analytical functions and working with large databases), as well as specific databases (MySQL, Oracle Database, SQL Server) and features of the implementation of certain SQL functionality on these servers. The book is ideal as a textbook for a novice database developer. It describes all possible uses of SQL and the most common database servers.