* * *

Can you state why this is the case? Are there products that need to use both?

I believe the assertion is that between those two youve accounted for a significant majority of the usage of SQL in the world.

Agreed. But aside from that I could easily see someone using PostgreSQL on their server, and storing data client side in SQLite

PostgreSQL's documented position indicates the standard does matter. Conformance to the standard is described as a "target" rather than PosrtgreSQL's feature set as being "defining". This stands to reason: you can't conform to a target without the target pre-existing and you'd already be conformant if you were setting the standard.

PostgreSQL's current documentation states:

"PostgreSQL development aims for conformance with the latest official version of the standard where such conformance does not contradict traditional features or common sense." -- https://www.postgresql.org/docs/15/features.html

This basic stance is documented at least as far back as PostgreSQL 7.3:

"PostgreSQL development tends to aim for conformance with the latest official version of the standard where such conformance does not contradict traditional features or common sense. At the time of this writing, ballotting is under way for a new revision of the standard, which, if approved, will eventually become the conformance target for future PostgreSQL development." -- https://www.postgresql.org/docs/7.3/features.html

I can only assume that the person you're responding to is advocating a personal opinion on what should count as the standard setting process for SQL (be that formal or informal). That person's opinion does not appear to be shared by the PostgreSQL team as if they believed that... they'd be conformant by definition and there would be no need to accommodate the standard. If they didn't care about the standard, presumably it wouldn't be listed as an "aim" in any capacity.

from the docs:

"Out of 177 mandatory features required for full Core conformance, PostgreSQL conforms to at least 170."

[1] https://www.postgresql.org/docs/current/features.html

A bit meta on my part, but I'd recommend referencing the version specific URLs from the PostgreSQL documentation when backing up a quoted claim. Your quote will referenceable now, but anyone seeing it later may see different number or the statement might be omitted from whatever the "current" version is in the future.

So instead of:

https://www.postgresql.org/docs/current/features.html

I'd recommend:

https://www.postgresql.org/docs/15/features.html

Yes.

Though it takes liberties whenever it feels the standard chosen something poorly.

Random examples:

* Standard says FKs are deferred by default. PostgreSQL uses immediate by default.

* Standards says triggers are executed in the order added. PostgreSQL executes them in alphabetical orders.