XQuery/Validating a hierarchy

Whilst schema validation can check for some aspects of model validity, business rules are often more complex than is expressible in XML schema. XQuery is a powerful language for describing more complex rules. One such rule is that a relationship should define a tree structure,for example, the relationship between an employee and her manager. Consider the following set of employees: <company> <emp> <name>Fred</name> <mgr>Bill</mgr> </emp> <emp> <name>Joe</name> <mgr>Bill</mgr> </emp> <emp> <name>Alice</name> <mgr>Joe</mgr> </emp> <emp> <name>Bill</name> </emp> </company> The criteria for a valid hierarchy are: one root (the boss); every employee has at most one manager; every employee reports finally to the boss; there are no cycles In XQuery we can define the management hierarchy from the boss down to an employee as : declare function local:management($emp as element(emp) , $hierarchy as element(emp)* ) as element(emp)* { if ($emp = $hierarchy ) (: cycle detected :) then () else let $mgr := $emp/../emp[name=$emp/mgr] return if (count($mgr) > 1) then () else if (empty ($mgr)) (: reached the root :) then ($emp,$hierarchy) else local:management($mgr, ($emp,$hierarchy)) }; The function is initially called as local:managment($emp,()) The hierarchy is built up as a parameter to allow cycles to be detected. Finally, the condition for the management structure to be a tree is declare function local:management-is-tree($company)) { let $boss := $company/emp[empty(mgr)] return count($boss) = 1 and (every $emp in $company/emp satisfies $boss = local:management($emp,())[1] ) };