SEMANTIC DATA MODEL TO RELATIONAL DATA MODEL - Database Design and Development: An Essential Guide for IT Professionals - page 276

SocSecNo

EmpName

EMPLOYEE

EmpID

EmpAddr

Primary
Key

Relational Notation EMPLOYEE (EmpID, SocSecNo, EmpName, EmpAddr)

EMPLOYEE
relation

EmpID

SocSecNo EmpName EmpAddr

1234 101-38-2121 Daniel R. Grady 6 William Street, Jamesburg, NJ 08810

2345 320-55-9641 Alice M. Knox 1 Bradford Place, Iselin, NJ 08834

3456 333-22-1111 Matthew Rader 47 Woods Drive, Edison, NJ 08817

Ramola Ruskin

4567 123-47-3257 55 Walker Lane, Metuchen, NJ 08819

Figure 9-5 Mapping of instance identifiers.

• In the transformation, nothing is expressed about the order of the columns in
the transformed relation.
• A single-valued or a derived attribute becomes one column in the resulting
relation.
• If a multivalued attribute is present, this is handled by forming a separate rela-
tion with this attribute as a column in the separate relation.
• For a composite attribute, as many columns are used as the number of com-
ponent attributes.

Instance Identifiers

In the semantic data model, each instance is uniquely identified by values in one or
more attributes. These attributes together form the instance identifier. Figure 9-5
indicates the transformation of instance identifiers.
Note the following points on this transformation:

• The set of attributes forming the instance identifier becomes the primary key
of the relation.
• If there is more than one attribute, all the corresponding columns are indicated
as primary key columns.
• Because the primary key columns represent instance identifiers, the combined
values in these columns for each row are unique.
• No two rows in the relation can have the same values in the primary key
columns.
• Because instance identifiers cannot have null values, no part of the primary key
columns can have null values.