SQLite is the fast ad small database, ideal for small or big application (web, mobile, server or desktop).
Don't require a server software and the DB file is portable to other platforms (Windows, UNIX, etc).
Most popular software use SQLite (eg. yum repositories, Firefox, Tenable, Android, etc.)
Cover most of SQL syntax and it's possible to create table, view, index, virtual table (eg. from a csv file or from a directory) and custom functions (via programming language).
SQLite is supported by PHP5, Perl, C, Tcl, C++, C#, Python, Rust, and more programming language.