This spring I took Introduction to Databases a MOOC (Massive Open Online Class) from Stanford’s Jennifer Widom. It was very interesting, I learned a great deal from it. And one of the sections of the class that I really enjoyed, even though it sounded like it was going to be pretty dry, was the section on XML. I took quite a few pages of notes on the different XMl tools, so I figured I might as well post them to my blog.

XPath Definition

XPath is a query language for selecting nodes from an XML document.

from Wikipedia

XML for the Examples

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<countries>
  <country name="Afghanistan" population="22664136" area="647500">
    <language percentage="11">Turkic</language>
    <language percentage="35">Pashtu</language>
    <language percentage="50">Afghan Persian</language>
  </country>
  <country name="Albania" population="3249136" area="28750"/>
  <country name="Algeria" population="29183032" area="2381740">
    <city>
      <name>Algiers</name>
      <population>1507241</population>
    </city>
  </country>
</countries>

Basic Path

Selects all of countries from the country root node.

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doc('countries.xml')/countries/country

Path with Alternation

This will select countries with populations that speak Pashtu or Swedish.

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doc('countries.xml')/countries/country[language(Pashtu|Swedish)]

Attributes are prepended with “@”

This will select the names of all countries

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doc('countries.xml')/countries/country/@name

Path with Conditional

Select the names of all countries that have a population of less than 1,000,000.

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doc('countries.xml')/countries/country[@population < 1000000]/@name

Complex Conditional

Select all countries that have a population of less than 1,000,000.

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doc('countries.xml')/countries/country[@population < 1000000 and city/population > 50000]

data() prints out just the text value.

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doc('countries.xml')/countries/country[@population < 1000000]/data(@name)