If SQL was a declarative language, there would not be expression in it, for example SELECT (1+2) , only SELECT 3
These categories based on the approach of the programming, not the underlying technology.
Imperative languages (high level): a step-by-step description of a process, like giving orders to someone/something in a sequence, organized further with loops and conditions.
Analogue: cooking recipe. Languages: C, Pascal, Basic, etc (they can be structured or object oriented, still imperative)
Functional language (not mathematical definition, forget stateless, non-mutable etc for a moment): It is an approach that solves the problem of cardinality of the imperative approach. There is nothing which is "one", only "one of them". It's a little bit confusing how i try to define this, but this is the most important aspect of this. We only filter and transform elements of a set, and even if this is nothing (for not mathematical aspect), but syntax sugar, it gives a programmer a safe way to do things, without caring about the details of enumerating things or null checks, etc. Or another angle: functional programming eliminates loops, and put the sequence in the data, and keep conditions. It is really handy to be honest. Purely functional languages are mostly experiments, because making something functional is kinda self-motivated, but other languages will pick up more functional elements in time, which is great.
Analogue: assembly line in a factory. Thing are coming, they are changed (like car body painted) or removed (like quality control). Languages: LINQ, SQL, F#
Declarative language: your approach here is instead of describing how to do something, you only describe what you want as a result. CSS is a perfect example, however it is picking up non-declarative elements nowadays, original it was only capable to describe how a font a paragraph look like. HTML is also declarative. Instead of actually drawing a rectangle, you just say, this width, height on this position, etc.
Analogue: you order a coffee in coffee shop. You don't care any of the details, just the parameters of the coffee you want to drink. Languages: HTML, CSS
Of course there are blurred lines like you can say all high level language are declarative, because you never go down to the hardware level to manually do everything, but I think what is important the approach, how you start to solve the problem, and if we see this from this point, SQL cannot be a declarative language. It was designed to be as close to English as possible to make non-IT people do programming, but it successfully failed it's purpose, because it is a pretty good language for programmers, but it was never designed to be declarative.
SQL is I think generally considered not entirely declarative, but that is not an example that shows that. Is there any declarative language that satisfies that, that has no addition (or operator? Or addition of constants? I'm not really clear what about it you think makes it not declarative?).
By this rule, Prolog and HCL/Terraform are not declarative either.
i give you other angles.
imperative has structures like: sequence (by code), loops and conditions
functional
** has no loop, the loop itself the cardinality, which is always multiple
** has no sequence, it's encoded by data (edit here, actually it has)
** has (of course) condition
So, by basically we redefine the fundamental elements of the imperative model
Purely declarative languages has no sequence/loop or conditions (in programming understanding)
Answering to you, HTML (especially earlier) is pretty close to purely declarative, i does not have loops or conditions, however, this is not 100% true, but close. I don't know those languages you mentioned, so i can not have an opinion about them
SQL fits this perfectly. For example JOINs don't describe actions, they describe relationships. Have you ever looked at what the query planners come up with to satisfy those relationships?
SQL does not fit my definition, because you reach your goals through multiple transformation and filtering, and this is how you reach your goal. you define the way, the process, not the end result. under there are some comments where i speak about this.
