Database Reference
In-Depth Information
Owner
record type
Member
record type
Owner
record type
CUSTOMER
SALES
TERRITORY
Owner
record type
Member
record type
ORDER
PAYMENT
Member
record type
SALES
PERSON
Owner
record type
Member
record type
Owner
record type
ORDER
LINE ITEM
Relationship links
through physical
pointers
Member
record type
Figure 1-9
Network data model.
The network data model overcomes some of the limitations of the hierarchical
data model. The network data model is more representative of real-world infor-
mation requirements than the hierarchical model. The network data model can
represent most business information.
Let us go over the key features of the network model by referring to Figure 1-9.
Levels.
As in most real-world situations, no hierarchical levels exist in the network
model. The lines in a network data model simply connect the appropriate data struc-
tures wherever necessary without the restriction of connecting only successive levels
as in the hierarchical model. Note the lines connecting the various data structures
with no restrictions.
Record Types.
In the network data model, each data structure is known as
a record type. For example, the CUSTOMER record type represents the data
content of all customers. The ORDER record type represents the data content of
all orders.
Relationships.
The network data model expresses relationships between two
record types by designating one as the owner record type and the other as the
member record type. For each occurrence of an owner record type, there are one
or more occurrences of the member record type. The owner record type may be
reckoned as the parent and the member record type as the child. In a sense, the
owner record type “owns” the corresponding member record type. Each member
type with its corresponding owner record type is known as a set. A set represents
the relationship between an owner and a member record type.
Multiple Parents.
Look at the ORDER member record type. For ORDER there
are two parents or owner records, namely, CUSTOMER and PAYMENT. In other
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