This is a database written in Prolog: https://github.com/terminusdb/terminus-server - albeit with a RUST storage engine - prolog's lack of types means there are better storage solutions, but for constraint logic, it's unbeatable.
Mercury is cool. I translated a simple Prolog program to it once, and it was a head trip. Hard to get my mind around, but also helped me see the connection between functions and relations.
We evaluated the use of Mercury before settling on prolog for terminusDB. Mercury is a great language, but the community and library support is just a bit too small.