Schema Translation - Information Systems Reengineering, Integration and Normalization - page 87

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Fig. 3.6 Map unary 1:n relationship from network to EER model

Substep 3—Derive unary relationships.
We map link (dummy) records of network schema into unary relationships. These
dummy records are either without any attributes, or contain key attributes only as
shown in Fig. 3.6 . The default is a 1:n relationship between owner and member for
each Set record type, but user input is sought to confirm or modify this relationship
into a 1:1 or an isa relationship.
Substep 4—Derive binary relationships.
Next we map each SET into a relationship between the owner and member re-
cords, assuming a default 1:n cardinality. However, one record type can be a mem-
ber of more than one SET. Multiple memberships logically intersect the owner re-
cords of two (or more) SETs. A member record type with two owner record types
implies an m:n relationship between the two owner record types. The member re-
cord type becomes a relationship relation between the two owner records as shown
in Fig. 3.7 . The default is 1:n relationship between owner and member for each Set
record type, but user input is sought to confirm or modify this relationship into a 1:1
or an isa relationship.
Substep 5—Derive entities of n-ary relationships.
For multiple record types linking together through Sets, if there is a semantic to
associate them in a relationship, then they are mapped as an n-ary relationship in the
EER model. Two examples are shown in Fig. 3.8 .
In Fig. 3.8 , each skill-used relationship is associated with n employee(s), n
project(s), and n skill(s), which implies that employees use a wide range of differ-
ent skills on each project with which they are associated.
The default is a binary relationship as described in step 3. A knowledge acquisi-
tion system should be able to detect a possible n-ary relationship from the DDL of
the network schema. Again, user input is sought to confirm or modify this relation-
ship. The user must be aware that a mandatory binary relationship can be grouped
as an m:n or n-ary relationship depending on the semantics. Above all, any optional