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182 8 Query Languages

for $bmi in

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

where $bmi > 30

return $bmi

Thereturnclause corresponds to the SELECT clause in an SQL query.

Programming languages like Perl also use the word “return,” but the

meaning is completely different.

4. letclause. It is often convenient to do computations in a query. To sim-
   plify the computations, it is helpful to be able to introduce variables that
   are set to expressions using the other variables. This is done usinglet
   clauses. There is no analogous capability in SQL. For example, the follow-
   ing query has the same result as the one above:

let $bmilist :=

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

for $bmi in $bmilist

where $bmi > 30

return $bmi

The$bmilistvariable is set to the whole collection ofBMIvalues. The

forclause then sets$bmito each of the values in this collection, one at a

time.

XQuery uses variables in ways that are different from how they are used

in programming languages such as Perl. In Perl, the dollar sign is used to

indicate that a variable is a scalar. A different symbol, the at-sign (@), is used

to indicate variables that can have an array of values. In XQuery, there is no

distinction between scalars and arrays: a variable can be either one. More

significantly, Perl variables can be assigned to a value any number of times.

XQuery will only assign a variable to different values in aforclause. A

variable can only be given a value once by aletclause. Subsequentlets

for the same variable are not allowed.

One can build up more complicated XQuery expressions by combining a

series offorandletclauses. These are followed by an optionalwhere

clause. Thereturnclause is always the last clause. For example, suppose

that one wants to obtain the major topics of the Medline articles in a corpus

of Medline citations. One would use a query like this: