Thank you Christian and Wray
The implicit cast is the problem (as usual). I was expecting it to happen equally :(
And I teach my students to use value comparisons when ever possible and to care about the cast yourself ;-)
Yours Leo
On 13 Dec 2018, at 10:46, Christian Grün christian.gruen@gmail.com wrote:
Hi Leo,
The semantics of value comparisons (eq, ne, …) and general comparisons (=, !=, …) are different. As E. Wray indicated, sequences with more than one item can be compared with general comparisons; the result will always be a boolean (value comparisons will return an empty sequence if at least one value is an empty sequence).
In addition, values of document nodes (in your case, @population) will be cast to the type of the other value (in your case, a double, resulting from fn:max). This implicit cast doesn’t take place if you use value comparisons, you’ll have to do this explicitly:
Does not work: <x a='1'/>[@a eq 1] Works: <x a='1'/>[number(@a) eq 1]
This is why your second query yields an error. See Sections 3.7.1 and 3.7.2 of the XQuery spec for all the comprehensive details [1].
In practice, I would recommend you to always use the general comparison operator in BaseX, as it provides more optimizations. In many cases, if it’s safe, value comparisons will automatically be rewritten to general comparisons by the compiler.
Best, Christian
[1] https://www.w3.org/TR/xquery-31/#id-value-comparisons
On Thu, Dec 13, 2018 at 10:17 AM Leo Studer leo.studer@varioweb.ch wrote:
Hello
Could someone explain me why this woks
//country[@population = max(//@population)]/name
Whereas this does not
//country[@population eq max(//@population)]/name
Thanks in advance Leo