The Dataphor stuff was all open sourced I believe. But it's in a C#/MS ecosystem, and tied into their overall application builder tool etc. Not really a standlone product.
There's other "D" type projects. Rel is one. Don't have URL handy but it's fairly easy to find. They publish their grammar and example code.
None of these tools have ever matured or become popular. I have theories why, we could discuss forever.
I think the issue has more to do with two things: a) most people don't understand the relational model fully, so they have no idea what they're missing b) new databases (and existing) simply cannot afford to rock the boat here because they need customers. And in terms of the architecture of the DB system, the SQL parser is one of the lower effort items (when compared to query planner, optimization, storage impl and storage optimization, replication, etc.) So why invest there for little value? Customers aren't asking for it.
The company I'm contracting for right now has some interest in working in this space.
Date & Darwin did good work with "The Third Manifesto" but it went almost completely ignored, and the tone and target of it may have been off. Frustratingly we went through a phase where SQL went out of style and then back into style ("NewSQL") and so there may have been a window missed there where alternative query languages (but still based on the relational model) could have risen. But instead "NoSQL" was too interested in jettisoning the relational model along with SQL (mostly I would argue because they don't understand it).
There has been some recent rise in Datalog implementations in the Clojure community. That is interesting, though not strictly as an alternative to SQL.
There's other "D" type projects. Rel is one. Don't have URL handy but it's fairly easy to find. They publish their grammar and example code.
None of these tools have ever matured or become popular. I have theories why, we could discuss forever.
I think the issue has more to do with two things: a) most people don't understand the relational model fully, so they have no idea what they're missing b) new databases (and existing) simply cannot afford to rock the boat here because they need customers. And in terms of the architecture of the DB system, the SQL parser is one of the lower effort items (when compared to query planner, optimization, storage impl and storage optimization, replication, etc.) So why invest there for little value? Customers aren't asking for it.
The company I'm contracting for right now has some interest in working in this space.
Date & Darwin did good work with "The Third Manifesto" but it went almost completely ignored, and the tone and target of it may have been off. Frustratingly we went through a phase where SQL went out of style and then back into style ("NewSQL") and so there may have been a window missed there where alternative query languages (but still based on the relational model) could have risen. But instead "NoSQL" was too interested in jettisoning the relational model along with SQL (mostly I would argue because they don't understand it).
There has been some recent rise in Datalog implementations in the Clojure community. That is interesting, though not strictly as an alternative to SQL.