Database Reference
In-Depth Information
Relational data model
Relations or Tables
Rows
Columns
Primary Key
Foreign Key
Generalization/Specialization
Mapping and Transformation
Just by going through the list of components, it is easy to form the basic concepts
for mapping and transformation. The semantic data model deals with the things that
are of interest to the organization, the characteristics of these things, and the rela-
tionships among these things. On the other hand, the relational model stipulates
how data about the things of interest must be perceived and stored, how the char-
acteristics must be represented, and how the links between related things must be
established.
First, let us consider the mapping of things and their characteristics. Then we will
move on to the discussion of relationships. As you know, a major strength of the
relational model is the way it represents relationships through logical links. We will
describe the mapping of relationships in detail and also take up special conditions.
Mapping involves taking the components of the semantic data model one by one
and finding the corresponding component or components in the relational data
model.
Object Sets to Relations
Let us begin with the most obvious component—the object set in the object-based
data model or the entity type in the entity-relationship data model. What is an object
set? If employee is a “thing” the organization is interested in storing information
about, then employee is an object represented in the semantic data model. The set
of all employees in the organization about whom data must be captured in the pro-
posed relational database system is the object set EMPLOYEE.
Figure 9-3 shows the mapping of the object set EMPLOYEE. The mapping shows
the transformation of the object set represented in object-based data modeling nota-
tion to the relation denoted in relational data model notation.
From the figure, note the following points about the transformation from object-
based data model to relational data model:
The object set is transformed into a relation.
The name of the object set becomes the name of the relation.
The object instances viewed as present inside the object set box transform into
the rows of the relation.
The complete set of object instances becomes the total set of rows of the rela-
tion or table.
In the transformation, nothing is expressed about the order of the rows in the
transformed relation.
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