Databases Reference
In-Depth Information
the extraction of the children of a node that are in given positions. Func-
tion
last()
evaluates to true on the last child of the current node. Function
position()
evaluates to true on the node in the evaluation context whose
position is equal to the context position.
Path expressions are also the building blocks of other languages, such as
XQuery [4] that allows to make queries on XML documents through
FLWOR
expressions. A
FLOWR
expression is composed of the following clauses:
•
FOR
declares variables that are iteratively associated with elements in the
XML documents, which are identified via path expressions;
•
LET
declares variables associated with the result of a path expression;
•
WHERE
imposes conditions on tuples;
•
ORDER BY
orders the result obtained by
FOR
and
LET
clauses;
•
RETURN
generates the final result returned to the requester.
Example 2.
Consider the DTD and the XML document in Example 1. Some
examples of path expressions are the following.
•
/account operation/operation
: returns the content of the
operation
element, child of
account operation
;
•
/account operation/@bankAccN
: returns attribute
bankAccN
of element
account operation
;
•
/account operation//notes
: returns the content of the
notes
el-
ements, anywhere in the subtree rooted at
account operation
;in
this case, it returns both
/account operation/request/notes
and
/account operation/operation/notes
;
•
/account operation/operation[./type=
“bank transfer”
]
: returns the
content of the
operation
element, child of
account operation
, only if
the
type
element, child of
operation
, has value equal to “bank transfer”.
The following XQuery extracts form the XML document in Fig. 1(b) all the
account operation
elements with operation type equal to “bank transfer”.
For the selected elements, the
amount
and the
recipient
of the operation are
returned, along with all
notes
appearing in the selected
account operation
element.
<
BankTransf
>
{
FOR
$r
in document
(“update account”)
/account operation
WHERE
$r
/operation/type
=“bank transfer”
RETURN
$r
/operation/amount
,$r
/operation/recipient
,$r
//notes
}
<
/BankTransf
>
3 XML Access Control Requirements
Due to the peculiar characteristics of the XML documents, they cannot be
protected by simply adopting traditional access control models, and specific
