Firebird RDBMS offers ANSI SQL features & runs on Linux, Windows & several Unix platforms. Features excellent concurrency & performance & powerful stored procedure & trigger language.
More information and downloads at https://firebirdsql.org/, issue tracker is https://github.com/FirebirdSQL/firebird/issues
Rel is a database management system (DBMS) that implements Tutorial D.
Rel is a DBMS primarily intended for educational purposes, but also suitable as a desktop DBMS or lightweight server, that implements Date and Darwen's "Tutorial D" true relational database language.
Source code is available at https://github.com/DaveVoorhis/Rel
The Technology Explorer for IBM DB2 is a light weight, web based console for DB2 that helps you learn about DB2 using demonstrations and tutorials. The TE is designed from the ground up to be easy to use and customize to suit your needs.