Re: Proposal: QUALIFY clause

"Matheus Alcantara" <matheusssilv97(at)gmail(dot)com> writes:
> You're right — semantically, using QUALIFY is equivalent to wrapping the
> query in a subquery and applying a WHERE clause to the result. The main
> motivation here is to provide a more ergonomic and readable syntax.

> While I understand the hesitation around introducing another keyword
> that effectively acts like WHERE at a different stage, I believe QUALIFY
> improves clarity in many use cases, by avoiding the boilerplate and
> visual noise of nested subqueries making it easier to write and reason
> about.

There are concrete reasons not to do this until/unless it becomes
standardized:

* If the syntax is like WHERE, there will be no way to do it without
making QUALIFY a fully-reserved word. That will inevitably break
more than zero applications. It's a lot easier to justify that
sort of breakage if we can say "QUALIFY is reserved according to
SQL:20xx, so don't blame us".

* I'm not exactly convinced that the committee would standardize
it just like this. For one thing, QUALIFY is not even the right
part of speech: it's a verb, and thus more fit to be a primary
statement keyword. What you need here is an adverb (I think ...
been a long time since high school English, but my dictionary
says WHERE is an adverb). Maybe they'd be persuaded to do what
the existing implementations did, but I wouldn't be at all surprised
if they choose a different keyword.

regards, tom lane