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176 8 Query Languages

8.1 XML Navigation Using XPath

XPath is a language for selecting parts of an XML document (W3C 1999). If

one has used computer file systems, then XPath navigation should be famil-

iar. For example, in the health study database, one can obtain all interviews

with this query:

HealthStudy/Interview

One specifies locations in an XML document by using the same notation that

is used for locating directories (folders) and files in most operating systems

(except that in Windows, a backward slash is used where a forward slash

would be used in XPath). Queries in XPath are calledpathsbecause they

describe the path to be followed to obtain the desired information.

In a document that has a much deeper hierarchical structure, one can use

a double slash to mean “skip any number of intermediate levels.” For ex-

ample, in the Medline database, one can obtain all substances by using this

path:

//NameOfSubstance

To obtain this set of elements without the double slash, one would have to

specify this path:

MedlineCitation/ChemicalList/Chemical/NameOfSubstance

Attributes are specified by using the at-sign character (@). Togetalistof

all of the body mass index (BMI) values in the health study, use this path:

//Interview/@BMI

The format of the result of a path will vary with the specific tool being used.

A typical result of the path above would look like this:

BMI = 18.66 BMI = 26.93 BMI = 33.95 BMI = 17.38

-> 4 item(s)

A path consists of a sequence ofsteps. Each step selects one or more desired

nodes. There are many kinds of node. The following are the most important:

1. element. To select an element, simply give its name. The name can in-
   clude a namespace prefix as in section 1.7. To select every element at one
   level, use an asterisk (*). The asterisk is also known as the “star” or the
   “wild card.” For example, in a MedlineCitation, one can obtain every
   child element of every Chemical node by using this path: