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8.2 Querying XML Using XQuery 181

will return all of the interview records in the health study database.
Alternatively, one can specify a collection of XML documents for which
one will perform a series of queries. Such a collection is called the “database”
orcorpus. The specification of the corpus will vary from one XQuery engine
to another. Once the corpus is ready, the queries do not have to mention any
documents.
So far we have only considered XPath expressions. Queries can be far
more elaborate. A general XQuery query may have four kinds of clause, as
follows:

1. forclause. This specifies a loop or iteration over a collection. It says that
   a variable is to take on each value in the collection, one value at a time.
   For example,

for $bmi in

document("healthstudy.xml")//Interview/@BMI

will set the$bmivariable to eachBMIattribute. All variables in XQuery

start with a dollar sign. This clause corresponds to the FROM clause in

SQL queries, except that in SQL one has only one FROM clause, while an

XQuery expression can have any number offorclauses. Most program-

ming languages (including Perl, C, C++, and Java) use “for” to indicate

an iteration process, and the meaning is the same.

2. whereclause. This restricts which values are to be included in the result
   of the query. This clause corresponds to thewhereclause in SQL queries.
   For example, if one were only interested inBMIvalues larger than 30, then
   the query would be

for $bmi in

document("healthstudy.xml")//Interview/@BMI

where $bmi > 30

Programming languages like Perl prefer to use the wordifto indicate a

restriction.

3. returnclause. A query can have any number of variables. The ones
   that should be printed are specified in thereturnclause. All of the
   queries given so far are actually incomplete, since they did not indicate
   what should be printed. To print theBMIvalues, the query above would
   look like this: