SQL's fine on its own. The problem arises when you try to manipulate it. A query to get messages that were sent between now and a week ago, joining in the user table for both the sender and the receiver to get their full names, is sensible enough when written out as SQL. I can quibble about some of the affordances of SQL, but it's good enough for most queries. The problem is if you try to create some sort of data that represents "get the message body", "messages sent since last week", "get the sender's name", "get the receiver's name", and somehow programmatically assemble a query out of the pieces. (Slight quibble, this isn't actually about OO, this is about "anything that isn't SQL". A Haskell library will have the exact same problem.) An AST-based approach doesn't guarantee that this will be easy, but it's still a step above trying to assemble an SQL query from those pieces.

It's totally possible. There's a ton of libraries that do it in various ways. It's just not even remotely easy, like it should be. If you look into the inside of those libraries you'll find a C'thulian monstronsity of special cases interacting with special cases until the whole thing just explodes into a brain-consuming mess, because SQL was very, very clearly not written for this use case.

In another 10 or 20 years I look forward to data-based analysis that tries to determine how much of the "NoSQL" movement was because the relational data model doesn't work for all use cases, and how much of it was the entirely accidental (in the Brooks sense) problems with A: SQL, the language itself, not its capabilities and B: schema migrations with no essential reason to be as painful as they are. (And something something column stores, but I'm not sure where they fit into this story exactly.) And to be clear on tone, I really am interested. I'm pretty sure the answer won't be either extreme but I'm pretty uncertain about where in the middle we'll fall.

I should clarify, I'm a big fan of SQL and relational DBs, but it's hard to deny that some/many devs find the abstraction boundary a bit weird.

And really, when you think about it, it is a bit weird, (we send sentences of almost-english toward the DB, then get structured data back) and there's nothing wrong with that.

But many devs do value the ability to stay in hash-map land all day without needing to think about how to cross an abstraction boundary into the DB, and MongoDB actually has a pretty cool solution there.

SQL is fine, no problem with it unless you are embedding queries in a OO application and you are forced to hack mapping layer between the two interfaces, which gives you two options, either compromise performance for the sake of development easiness or vice versa.