Legacy System Models: Hierarchical and Network - Database Design and Development: An Essential Guide for IT Professionals - page 678

Essentially, the network data model is based on the 1971 DBTG report. This data
model conforms to a three-level database architecture: conceptual, external, and
internal levels. A number of commercial database systems were developed to imple-
ment the network data model.
Summary of Basic Concepts
• Data is organized in the form of records being arranged as a network of nodes.
• Two fundamental modeling concepts make up the network data model: record
types and set.
• Two record types are linked as a set. The set expresses the one-to-one or one-
to-many relationship between two record types.
• A set expressing the relationship between two record types consists of a
member record type and an owner record type.
• One owner record type may be part of different sets with different member
record types.
• Similarly, one member record type may have multiple owner record types.
• A network consisting of one-to-one or one-to-many relationships is known as
a simple network. A complex network, on the other hand, contains many-to-
many relationships also.
• Each record type generally represents an entity type of the organization. Data
fields in the segment types denote the attributes of the entity type.
• An instance of a set type represents one occurrence of the entity represented
by the record type.
• Logical links between related records are implemented through physical
addresses (pointers) in the record itself.
Semantic to Network Data Model
The following indicates the mapping of components of the semantic data model into
components of the network data model.
Object Sets
• Object set is transformed into record type.
• The name of the object set becomes the name of the segment type.
CUSTOMER object is transformed into CUSTOMER record type.
• The object instances of an object set become the occurrences of the trans-
formed record type.
• The complete set of object instances becomes the total set of record
occurrences.
Attributes
• Attributes of an object are transformed into the data fields of the field type.
• The names of the attributes become the names of the data fields.