Just to say there is "Project:M36 Relational Algebra Engine" [1], which says about itself: Introduction Project:M36 implements a relational algebra engine as

Just to say there is "Project:M36 Relational Algebra Engine" [1], which says about itself: Introduction Project:M36 implements a relational algebra engine as inspired by the writings of Chris Date. Description Unlike most database management systems (DBMS), Project:M36 is opinionated software which adheres strictly to the mathematics of the relational algebra. The purpose of this adherence is to prove that software which implements mathematically-sound design principles reaps benefits in the form of code clarity, consistency, performance, and future-proofing. Project:M36 can be used as an in-process or remote DBMS. 1: https://github.com/agentm/project-m36 https://github.com/agentm/project-m36