Database Reference
In-Depth Information
Fig. 9.14 Relational database schema (primary key is underlined and foreign key is marked by “#”)
attribute, ce indicates if
is a primary
key or not, re is the relation that contains the exported primary key, and f a is the
attribute name of the foreign key.
b
is a foreign key or not, cp indicates if
b
This schema provides an image of metadata obtained from an existing relational
database and provides more information than does a traditional schema. Indeed, it
gives information about primary and foreign keys to facilitate the extraction of
relations in subsequent steps. For example, for the Table “ Thesis ” (database in
Fig. 9.14 ), the relation Thesis ( th_id, Th_name, Topic) was extracted. Once the
schema has been extracted, we can generate the corresponding hypernode schema
(step 2).
Step 2: Mapping the relational schema to the hypernode schema . We use for this
step a hypernode schema, which is an extension of the original one.
A hypernode is defined [ 25 ]by H
( N, E ), where N is a finite set of nodes
containing primitive nodes and further hypernodes, and E is a set of edges between
members of N , such that N
¼
A
[
L (where A is the set of atomic values and L the
set of labels) and E
.
A Hypernode Database ( HD ) is a finite set of hypernodes, which satisfies the
following conditions:
ð
N
N
Þ
1. The hypernode label is unique in HD .
2.
8
H a label in the label set of HD ,
9
h
HD whose defining label is H.
The Hypernode model does not use labeled edges; the task of representing
relations (and their names) can be accomplished by encapsulating edges, which
represent the same relation (same label edges), within one hypernode labeled with
the relation name. However, the traditional presentation of a social network is a
labeled node attached to an explicit labeled edge. Then, we extend the HD to LHD
(Labeled Hypernode Database) by adding explicit labels to edges. LHD
¼
HS
\
ES ,
where:
1. HS is a finite set of hypernode.
2. ES is a set of edge where ES
ð
HS
HS
Þ and 8
e
2
ES , e has a label.
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