That is really fascinating, I will have a good look at that, thanks!
I have a SQL parser in Haskell on github here (using Parsec):
https://github.com/JakeWheat/hssqlppp
The model in the article is roughly based on the version in Date and Darwen's work which is almost the same as what you mention: project, rename, extend, join, union and not matching. The summarize by is kind of syntax sugar in this system. I left out some of the operators which weren't used anywhere in the code in the article. Industrial grade conversion from SQL to relational algebra is a real challenge!
Wow, your parser is a behemoth! My parser ended up being around 500 lines of Yacc. I know I didn't fully implement SQL, but I thought I had gotten pretty close. Are you doing any extra fancy stuff inside of the parser? Clearly the documentation is pretty extensive. I like the lhs work; I might start doing some of my projects in LHS.
SQL to RA is definitely quite a task, but it was pretty fun (up until the end, when it just started getting annoying...). Then again, I think compilers and programming languages are a blast anyway. Haskell makes it even more so :)
When I wrote this code, I wasn't really comfortable using monads, so all of the threading is explicit. If I were to revisit it, I would probably use a state monad, which might also make it easier to make it tail-recursive (as it is, my desugarer is not written in a tail-recursive manner).
The model in the article is roughly based on the version in Date and Darwen's work which is almost the same as what you mention: project, rename, extend, join, union and not matching. The summarize by is kind of syntax sugar in this system. I left out some of the operators which weren't used anywhere in the code in the article. Industrial grade conversion from SQL to relational algebra is a real challenge!