Why is $node/@attr != 'blue' false when $node is null?

> The code below returns 'blue', but I was expecting 'not blue'. The class > attribute on an empty node is null; null != blue. > > let $node := () > return > if($node/@class != 'blue') > then 'not blue' > else 'blue' > > > -- > France Baril > Architecte documentaire / Documentation architect > france.baril at architextus.com

On Fri, Jul 10, 2015 at 12:07 AM, Christian Grün <christian.gruen at gmail.com> wrote: > Hi France, > > The semantics of general comparisons in XQuery (==, != etc.) is > "existential". Each item of a sequence is compared against each item > of the other sequence. If a single test returns true, the final result > is true (and the remaining comparisons are usually skipped). > Otherwise, the result if false. If one of the sequences is empty, > there is nothing to compare, and the result will always be false: > > 1 = 1 → true > (2,3) = (1,2) → true > 1 = () → false > () = () → false > () != () → false > > Inverting the operator, and inverting the if branches or using the > not() function helps you out: > > let $node := () > return > if(not($node/@class = 'blue')) > then 'not blue' > else 'blue' > > Best, > Christian > > > > The code below returns 'blue', but I was expecting 'not blue'. The class > > attribute on an empty node is null; null != blue. > > > > let $node := () > > return > > if($node/@class != 'blue') > > then 'not blue' > > else 'blue' > > > > > > -- > > France Baril > > Architecte documentaire / Documentation architect > > france.baril at architextus.com >