XQuery: How to know if there are doublets?

Question

I have XML file there is <a> and  for each element

I want to write a query using XQuery to return True or False

there is an element called <element>.

each <element> has 2 element in it <a>and.

Return False : if there is any <a> has the same value as another <a> in another element && there 's value are different

otherwise True : <a> values are differnt in each element or there is similarity but there  values are different

for example

<root>
<element>
   <a>ttt</a>
   <b>tttsame</b>
</element>
<element>
   <a>ttt</a>
   <b>tttsame</b>
</element>
<element>
   <a/>
   <b>value</b>
</element>
<element>
   <a>rrr</a>
   <b>rrrvalue</b>
</element>
<element>
   <a>mmm</a>
   <b>rrrvalue</b>
</element>
<element>
   <a>mmm</a>
   <b>rrrvalue</b>
</element>
</root>

This one should be okay should return true

<root>
<element>
   <a>ttt</a>
   <b>ttt value</b>
</element>
<element>
   <a>ttt</a>
   <b>ttrdiff</b>
</element>
<element>
   <a/>
   <b>value</b>
</element>
<element>
   <a>mmm</a>
   <b>rrrvalue</b>
</element> 
</root>

shoudn't be accepted because ttt has two different values should return false

This is still unclear. What's the difference between the "good" XML and the "bad" XML? — zx485
There is still no clear relation between input XML and output XML. — zx485

Martin Honnen Martin Honnen · Accepted Answer · 2019-10-19T08:26:48

You could group on a and then check if there is more than one distinct b in any group, for instance with

not
(
    for $a-group in root/element
    group by $a := $a-group/a
    where tail(distinct-values($a-group/b))
    return $a-group
)

https://xqueryfiddle.liberty-development.net/6qM2e2r/0 and https://xqueryfiddle.liberty-development.net/6qM2e2r/1 has your two input samples.

As for how it works, the question asks to return false "if there is any <a> has the same value as another <a> in another element && there 's value are different".

To find element elements with the same a child element we can group by $a := $a-group/a in a for $a-group in root/element expression. The distinct or different b values in each group of as with the same value are computed by distinct-values($a-group/b), if there are at least two different b values then tail(distinct-values($a-group/b)) contains at least one value, otherwise it is an empty sequence. This works as through XQuery 3's group by clause "In the post-grouping tuple generated for a given group, each non-grouping variable is bound to a sequence containing the concatenated values of that variable in all the pre-grouping tuples that were assigned to that group" (https://www.w3.org/TR/xquery-31/#id-group-by) so that after the group by $a := $a-group/a clause the variable $a-group is bound to a sequence of element elements with the same grouping key based on the a child element.

So the complete for .. group by .. where .. return selects the groups of element elements with the same a value where there are at least two different/distinct b values.

As the requirement is to "return false" if any such groups exist the not() function is applied to implement that condition as the boolean value of a non-empty sequence is true and the not(..) then gives false if there are any elements meeting the condition expressed in the for selection.

XQuery: How to know if there are doublets?

3 Answers