On Thursday, February 16th, 2023 at 5:13 PM, David G. Johnston
wrote:
> This is how we've chosen to document this kind of grammar and I don't see a
> compelling reason to revisit that decision on the basis of this report. I
> could consider some additional wording regarding "each clause keyword only
> ever appears once in a given (sub)query" though that seems almost
> self-evident as part of the learning of SQL. The fact that you write WINDOW
> and then a comma-separated listing of names plus definitions is how that
> syntax diagram is read, just like with WITH, FROM, and GROUP BY clauses in
> the same diagram.
Yes, such additional wording would be helpful, thanks, as well as would be an
example. Perhaps not in the SELECT reference section, but in section 4.2.8,
which has a lot of detail about using window functions. Section 4.2.8 does have
a single mention of existing_window_name in bold italics, as if it would be
described in more detail lower on the page, but that's the only mention of it
there.
The only instruction for how to have multiple window definitions is then in the
SELECT reference, where it is represented only by [, ... ] indicating
repetition of something, so that's good the reader at least knows it's
possible, even if it's unclear exactly how. Inferring wrongly what the correct
form is tripped me up, and it tripped up the guy who posted the question on SO,
and at least six others upvoted the answer there, so at least a few people find
value in being explicit about this.
Incidentally the other clauses you mention are all supported with examples in
the manual showing how to include multiple elements without repeating the
keyword: FROM in 2.6, WITH in 7.8.1, GROUP BY in 7.2.3. Besides the examples,
that section 7.2.3 provides the syntax for GROUP BY as:
GROUP BY
grouping_column_reference
[
,
grouping_column_reference
]...
with grouping_column_reference repeated to indicate clearly that there is no
keyword necessary before the second and subsequent elements, so at least in
some contexts the clarity provided by such repetition is appropriate.
Inconsistently the syntax given for WINDOW is:
WINDOW
window_name
AS (
window_definition
) [, ...]
rather than something like
WINDOW
window_name
AS (
window_definition
) [,
window_name
AS (
window_definition
)]...
Maybe the reference section for SELECT is the wrong place for that longer form,
but section 4.2.8 (or some other referenced from there) could be.
Cheers,
--
Adam Mackler
