[xquery-talk] distinct-values in Where Clauses

[Andrew Welch]

On 18/01/2008, Ronald Bourret <rpbourret at rpbourret.com> wrote:
> I'm not entirely sure what it is you want, but I have a few comments:
>
> 1) Why are you using nested for loops? The following clause:
>
>     for $find in doc("resume.xml")//div, $head in $find/head, $desc in
> $find/p
>     do something
>
> is similar to:
>
>     for i=1 to 10
>        for j=1 to 20
>           for k=1 to 30
>             do something
>
> That is, the code finds each div. For each div, it finds all the head
> children. And for each head child, it finds all the p children. It then
> executes the return clause once for each p child.

Just to add to that description - it's called the Cartesian product
and generates all possible combinations of the input values, eg:

for $a in 1 to 2, $b in ('a', 'b')
return concat($a, $b)

generates:

1a 1b 2a 2b

It might be easier to visualise when its two simple sequences like
that.  Generally I don't think you want to use a Cartesian product
unless you know what you're doing...  Maybe a seasoned xquery'er can
provide a couple of common use cases where a Cartesian product is the
ideal solution?

cheers
-- 
Andrew Welch
http://andrewjwelch.com
Kernow: http://kernowforsaxon.sf.net/