The book "Guide to Databases" tells about the architectural principles of modern DBMS, as well as the algorithms and data structures that are used in them. It will give practical knowledge a solid foundation in the form of understanding general patterns. Particular attention is paid to comparing implementations of the same approaches in platforms with similar functionality. In addition, an original classification of DBMS is proposed, issues of operation and safety are considered. The book is written for information systems architects and leading developers. At the beginning of the XXI century, the very concept of "database" implied precisely the relational base. Having appeared in the 70s of the XX century, relational platforms are still the most common and universal. In the ranking of popular databases maintained by the db-engines.com portal, the first four places are occupied by relational DBMSs - Oracle, MySQL, Microsoft SQL Server and PostgreSQL, and among the 50 most popular relational databases - more than half. Relational databases are difficult to scale horizontally, because looking at a query in SQL, it is impossible to guess where the data that this query will require is located. A new direction in the development of databases was called NoSQL, and the first NoSQL platforms were key-value repositories.