Database Modeling and Design

68 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling
schemas merged at one time and an n-ary approach with n schemas
merged at one time, where n is between 2 and the total number of schemas
developed in the conceptual design. The binary approach is attractive
because each merge involves a small number of data model constructs
and is easier to conceptualize. The n-ary approach may require
only one grand merge, but the number of constructs may be so large
that it is not humanly possible to organize the transformations properly.
4.4.2 Comparison of Schemas
In the second step, comparison of schemas, the designer looks at how
entities correspond and detects conflicts arising from schema diversity—
that is, from user groups adopting different viewpoints in their respective
schemas. Naming conflicts include synonyms and homonyms. Synonyms
occur when different names are given for the same concept;
these can be detected by scanning the data dictionary, if one has been
established for the database. Homonyms occur when the same name is
used for different concepts. These can only be detected by scanning the
different schemas and looking for common names.
Structural conflicts occur in the schema structure itself. Type conflicts
involve using different constructs to model the same concept. In
Figure 4.4, for example, an entity, a relationship, or an attribute can be
used to model the concept of order in a business database. Dependency
conflicts result when users specify different levels of connectivity (oneto-many,
etc.) for similar or even the same concepts. One way to resolve
such conflicts might be to use only the most general connectivity—for
example, many-to-many. If that is not semantically correct, change the
names of entities so that each type of connectivity has a different set of
entity names. Key conflicts occur when different keys are assigned to the
same entity in different views. For example, a key conflict occurs if an
employee’s full name, employee ID number, and Social Security number
are all assigned as keys.
4.4.3 Conformation of Schemas
The resolution of conflicts often requires user and designer interaction.
The basic goal of the third step is to align or conform schemas to make
them compatible for integration. The entities, as well as the key
attributes, may need to be renamed. Conversion may be required so that
concepts modeled as entities, attributes, or relationships are conformed
to be only one of them. Relationships with equal degree, roles, and con-