Database Reference
In-Depth Information
Figure 4. Transformations to simplify a DTD
node of a transition tree (the same tree if the DTD
contains a recursion). In addition, the arcs of the
transition tree can be labeled with the attribute
type (ID, IDREF).
R2. Each element that contains at least one non
PCDATA element is a root for a transition
tree.
Rule R2 excludes transition trees composed of
the root connected to only leaf nodes (PCDATA).
Such a tree, in the XML document, can be con-
sidered as a basic data entity, in the sense that it
cannot represent a relationship. In addition, by
imposing that a transition tree contains at least one
complex element, R2 ensures that the transition
tree represents a relationship.
Root Determination
We determine the root node of a transition tree by one
of the following four rules that we have developed
in (Hachaichi, Feki, & Ben-Abdallah, 2008-a):
R1. Each DTD element that does not appear in
the declaration of any other element is a
root of a transition tree.
R3. Each element contained in the declaration of
n elements ( n ≥ 2) is the root of a transition
tree.
In general, each XML document can be seen as
one root element that contains all the elements in
the document. Thus, rule R1 will extract this top-
most element as a root of one transition tree. The
application of R1 must however exclude recursive
DTD where all elements are sub-elements of other
elements (Yan, & ADA, 2001). We treat the case
of a recursive DTD in the rule R4 .
This rule avoids the redundancy of elements in
a transition tree and identifies the elements shared
by several trees as one transition tree.
R4. Each element directly or transitively con-
taining its own declaration is the root of a
transition tree.
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