It's readable - though obviously you're going to need experience in RDBMS theory (Codd's paper, etc) to grok it.
It is very verbose, though.
Fortunately, the only people who need the ISO SQL spec are the vendors: the people writing SQL engines and tooling: it's next-to-useless for people who are designing databases or writing queries against them, chiefly because no RDBMS implementation, ever, has come close to implementing the full specification, and everyone has their own extensions to SQL - so all the vendors put out their own documentation and the world's largely been happy with that for the past ~40 years that SQL's been relevant.
It's not too hard to find the standards in PDF form if you know where to look - and as someone who does a fair bit of SQL, I'll say I've only ever referred to the standards to back-up my more controversial posts on StackOverflow.
I'm heading in the direction which is engines/tooling and I'm amazed at the number of corners of SQL that I have discovered that I don't know properly. You can use it fine for decades but when you need to know precisely what is acceptable to the GROUP BY/HAVING clauseS... then you suddenly realise you don't. It's quite surprising.
it would be useful to have that standard. Thanks, will have a hunt.
In that case I'm curious what it is about GROUP BY and HAVING that you recently learned - everyone I know that groks division knows their RDBMS-of-choice's SQL dialect inside out - it's like it's impossible to learn one without the other.