THE XML REVOLUTION - TECHNOLOGIES FOR THE FUTURE WEB

FLWOR expressions

The main engine of XQuery is the FLWOR expression:

For-Let-Where-Order-Return
pronounced "flower"
generalizes SELECT-FROM-HAVING-WHERE from SQL

A complete example is:

for $d in document("depts.xml")//deptno
let $e := document("emps.xml")//employee[deptno = $d]
where count($e) >= 10
order by avg($e/salary) descending
return
  <big-dept>
     { $d,
        <headcount>{count($e)}</headcount>,
        <avgsal>{avg($e/salary)}</avgsal>
     }
  </big-dept>

for generates an ordered list of bindings of deptno values to $d
let associates to each binding a further binding of the list of emp elements with that department number to $e
at this stage, we have an ordered list of tuples of bindings: ($d,$e)
where filters that list to retain only the desired tuples
order sorts that list by the given criteria
return constructs for each tuple a resulting value

The combined result is in this case a list of departments with at least 10 employees, sorted by average salaries.

General rules:

for and let may be used many times in any order
only one where is allowed
many different sorting criteria can be specified

Note the difference between for and let:

for $x in /company/employee

generates a list of bindings of $x to each employee element in the company, but:

let $x := /company/employee

generates a single binding of $x to the list of employee elements in the company.

This is also sufficient to compute joins of documents:

for $p IN document("www.irs.gov/taxpayers.xml")//person for $n IN document("neighbors.xml")//neighbor[ssn = $p/ssn] return <person> <ssn> { $p/ssn } </ssn> { $n/name } <income> { $p/income } </income> </person>