Re: newby (very) question on XML DB theory

Mikito Harakiri wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:Eyhfc.71539$0O2.4854903_at_phobos.telenet-ops.be...
>

>>//Courses[starts-with(.//_at_instructor_last_name, "VAN")]
>>
>>I've taught this stuff to CS students and non-CS students. I have no
>>idea why you think this would be too difficult for the latter. I do,
>>however, have an idea why a non-declarative query language that requires
>>programming if queries get a little bit more difficult would be
>>problematic for them.

You may. :-) Seriously though, who has claimed otherwise?

> Take outer join for, example. As purity is broken (by allowing nulls into
> result set) selection and outer join operation don't commute anymore. It
> took me some time (with usenet help) to realize that
>
> select * from t1 left join t2 on t1.id=t2.id and t2.id=2
>
> is different from
>
> select * from t1 left join t2 on t1.id=t2.id where t2.id=2
>
> for example. This is never an issue with ordinary joins and selections.

I'm not sure that is a good example because null values (for XML: undefined attributes or missing elements) are actually dealt with in XQuery in a very clear and consistent manner. But if your point is that XML is a much more complicated data model and therefore its query language in general a much more complicated query language, then, yes, I would certainly agree.

> Simplicity of the underlying algebra is the key for the query language
> success. (Hmm, what about SQL?)

Actually the original definitions of semistructured data models were really quite simple and elegant, and the corresponding languages based on solid theory. But then XML came along and the whole thing got messy. Sounds familiar, no?

• Jan Hidders