> The example seems expressly permitted by the standard and other database > systems.
The example is illegal in SQL:2003 and before; they did not allow ORDER BY in a <query expression> until SQL:2008. Even in newer spec versions, it's considered an optional feature (cf F850-F855). Given that, I would be pretty leery of claims that it's supported in all other DBMSes. We should perhaps back off the wording to something like "is not allowed in older versions of the SQL standard, and may not be portable to other database systems".
I guess that is the only portion that would make sense to be illegal...
I won't claim to know the behavior other databases with respect to the allow-ability of an ORDER BY clause - I was looking at the <query expression> construct as a whole.
Based upon what you've said I would soften it a bit. Given my own experience I'd probably point out what is now obvious to me - that the allowance of the ORDER BY clause is implementation specific. But I'd be fine chalking that up to an anomalous reading.
Something like:
"But permitting the sub-query's ORDER BY was only upgraded to optional in SQL:2008 and thus this syntax poses a portability hazard."