xpath query returning wrong values when using greater and less than?

Question

This query is returning values less than 1000. It should only be returning values between 1000 and 1100. Why is that?

//results/Building[ 1 = 1 and (( Vacancy/sqft > 1000 ) and ( Vacancy/sqft < 1100 ) ) ]

The query will return the following building, which has vacancies less than 1000 square feet and greater than 1100 square feet:

<Building>
  <Vacancy><sqft>900</sqft></Vacancy>
  <Vacancy><sqft>1000</sqft></Vacancy>
  <Vacancy><sqft>2000</sqft></Vacancy>
  <Vacancy><sqft>500</sqft></Vacancy>
</Building>

Why is it included in the results?

Sample data:

<results>
  <Building><!--Shouldn't be selected.--></Building>

  <Building><!--Should be selected-->
    <Vacancy><sqft>1050</sqft></Vacancy>
  </Building>

  <Building><!--Should be selected-->
    <Vacancy><sqft>1025</sqft></Vacancy>
    <Vacancy><sqft>1075</sqft></Vacancy>
  </Building>

  <Building><!--Shouldn't be selected-->
    <Vacancy><sqft>10</sqft></Vacancy>
    <Vacancy><sqft>50</sqft></Vacancy>
  </Building>

  <Building><!--Should be selected.-->
    <Vacancy><sqft>1050</sqft></Vacancy>
    <Vacancy><sqft>2000</sqft></Vacancy>
  </Building>

  <Building><!--Should be selected.-->
    <Vacancy><sqft>900</sqft></Vacancy>
    <Vacancy><sqft>1040</sqft></Vacancy>
  </Building>

  <Building><!--Shouldn't be selected-->
    <Vacancy><sqft>10500</sqft></Vacancy>
  </Building>

  <Building><!--Shouldn't be selected-->
    <Vacancy><sqft>900</sqft></Vacancy>
    <Vacancy><sqft>1000</sqft></Vacancy>
    <Vacancy><sqft>2000</sqft></Vacancy>
    <Vacancy><sqft>500</sqft></Vacancy>
  </Building>

</results>

Thanks.

Answer 1

The sample Building has a Vacancy child with sqft of 2000, so Vacancy/sqft > 1000 succeeds. It has a child with sqft of 1000 (and 900 and 500), so Vacancy/sqft < 1100 succeeds. Thus the xpath selects the Building.

The comparison expressions (such as Vacancy/sqft <= 1000) are implicitly qualified with "there exists"–as in "there exists a Vacancy child that has a sqft child with value > 1000"–because Vacancy/sqft is a set of nodes, rather than a single node. Moreover, each comparison has its own qualification, so the sqft in Vacancy/sqft > 1000 doesn't need to be the same sqft as in Vacancy/sqft < 1100. Note that //results/Buildings is a node set; the predicate [...] applies separately to each item in the set, which is why there isn't an issue with qualifiers. Translating your original xpath into English, we get:

Select the buildings (in the results) such that 1=1 and there exists a vacancy square footage > 1000 and there exists a vacancy square footage < 1100.

Let's take the English statement of the desired query and make it a little closer to a statement of logic, arriving at one of:

Select the buildings (in the results) such that there exists a vacancy with square footage such that it's > 1000 and it's < 1100

Select the buildings (in the results) such that there exists a vacancy such that the square footage > 1000 and the square footage < 1100

The former leads to jasso's solution, the latter to:

//results/Building[ Vacancy[1000 < sqft and sqft < 1100] ]

Original solution

(Note: this answered the original question, when it wasn't clear what the OP wanted. The technique may prove useful to others with a similar problem but different requirements, so I'm leaving it in.)

Try the logical double-negation of the condition:

//results/Building[ Vacancy and not (Vacancy/sqft <= 1000 or Vacancy/sqft >= 1100) ]

This predicate includes a test for Vacancy children to filter out cases that are otherwise trivially true, i.e. buildings with no vacancies. The English equivalent of this solution is:

Select buildings (in the results) such that the building has a vacancy and it's not the case that there exists a vacancy square footage <= 1000 or there exists a vacancy square footage >= 1100

In fewer words:

Select all buildings with vacancies where no vacancy has <= 1000 square feet or >= 1100 square feet.

In fewer words still:

Select all buildings with vacancies where all vacancies are between 1000 and 1100 square feet.