C# LINQ is basically very roundabout way of doing what SQL was made for. What SQL does, and does really well, is operate on sets of data (aka tables) using set theory. You can get succinct expression with C# and functional languages, but they are not really possible to optimize the same way SQL is because it's not bound by set theory the same way.
SQL by default operates on multisets instead of sets; iIt has no syntax for a set (aka table) literal, SELECT 1, "a" FROM DUAL UNION SELECT 2, "b" UNION ... doesn't count; sub-queries (using another SELECT in a FROM clause) are a pretty recent addition; and the list goes on. It's not based on a set theory, or on the first-order logic, or the relational model, not really. If it were, it would be be Prolog.
Regarding table literals, some products support using
VALUES (1, 'a'), (2, 'b'), ...
as a table constructor in general, not just in INSERT, and looking at SQL:92, SQL:99, and SQL:2003 it looks to me like this should be standard SQL. Derived tables, aka subqueries, exist in SQL:92 at least as well, so whether or not that is considered recent depends on how you look at it, I think.
I am not trying to defend SQL with this, and all in all this does not take away from the points you raised, but the above were something that stood out.
They were in SQL:92, but IIRC adoption was somewhat slow and patchy, and has generally finished somewhen in the early 2000s.
My point is, the parent's claim that SQL "operate[s] on sets of data (aka tables) using set theory", and does it "really well" but when you actually look at SQL, you realise that sets/tables aren't really first-class ― derived tables were added in later versions of SQL, and literal tables still don't exist, but those are things you expect a language focused on table manipulations to have. Nope, it's a language for building very specific kinds of queries which was then patched and extended into something more general.
