While not an RDBMS altogether, Deklarit supplies SQL's deficiencies in constraints.

Java in-memory SQL92 DBMS with sane NULLs processing. Purports to actually strive to follow Codd's rules.

Another relational-like Unix simple files database.

A descendant of RDB.

Makers of a very simple quasi-relational database based on Unix shell commands.

The first implementation corrupted from the relational model, was the basis for SQL and DB2. A site preserving early history of a technology that became a critical business tool.

Another Unix filter implementation of some relational ideas.

A rules engine allowing declarative rules to be automatically enforce, thus implementing the constraints aspect of an RDBMS.