How to get all ungrouped items with distinct-values() in XQuery?

Question

With this XQuery expression

for $ex in distinct-values(//products/product/@group)
return $ex

and this XML

<xml>
    <products>
        <product id="10" group="2"></product>
        <product id="20" group="1"></product>
        <product id="25" group=""></product>
        <product id="30" group="2"></product>
        <product id="35" group=""></product>
    </products>
</xml>

I'm getting only:

• product group '2'
• product group '1'

But I also need:

• product group '' (@id=25)
• product group '' (@id=35)

Expecting (order not of interest):

Only 1 product (the first if more than 1 with the same group) of a group and also every product with no group.

• product id 10 with group '2'
• product id 20 with group '1'
• product id 35 with group ''
• product id 25 with group ''

Every product with an empty group should also be returned in this xquery, but how?

Answer 1

Also the empty string is a distinct value and should be returned already, but likely you're simply not observing it. Changing your query to

for $ex in distinct-values(//products/product/@group)
return <value>{ $ex }</value>

you will get a result like

<value>2</value>
<value>1</value>
<value/>

(while <value/> is equivalent to <value></value> , and "contains" the empty string).

If you want to match both a single product for empty groups and all products without a group, you can for example split apart the query in two parts: filtering for products with groups, and then adding those without.

(
  (: return first product for each product group :)
  for $group in distinct-values(//products/product/@group)
  (: unless group is empty :)
  where $group
  return (//product[@group eq $group])[1],
  (: add products without any group :)
  //product[@group eq '']
)

Answer 2

Xpath 1.0 gives an ability to find this elements without any program code

//product[
  not(@group = preceding-sibling::product/@group) 
  or (@group="")
  ]