Database Reference
In-Depth Information
one file and records in another file. The network approach provides more flexibility than
the hierarchical approach and allows a database designer to optimize the database using de-
tailed control and data organization. The network model allows child tables to have more
than one parent, thus creating a networked-like table structure.
A
node
represents a collection of records, and a
set structure
establishes and represents a re-
lationship in a network database. It is a transparent construction that relates a pair of nodes
together by using one node as an
owner
and the other node as a
member
. A user can access
data from within the network database, starting from any node and working backward or
forward through related sets. Multiple parent tables for each child allows for
many-to-many
relationships, in addition to one-to-many relationships.
A record in the member node cannot exist without being related to an existing record in
the owner node. One or more sets (connections) can be defined between a specific pair of
nodes, and a single node can also be involved in other sets with other nodes in the database.
The most popular database based on the network model was the Integrated Database Man-
agement System (IDMS), originally developed by Cullinane (later renamed Cullinet). The
product was enhanced with relational extensions, named IDMS/R and eventually sold to
Computer Associates.
Example 2:
In the network model contents example shown in figure, each parent-child relationship is
depicted with a different type of line, illustrating that each has a different name. This differ-
ence is important because it points out the largest downside of the network model, which is
complexity.
Instead of a single path that may be used for processing the records, there are now many
paths. To find all the other orders for this customer, there must be a way to work forward
from where we are to the end of the chain and then wrap around to the beginning and for-
ward from there until we return to the order from which we started. It is to satisfy this pro-
cessing need that all pointer chains in network model databases are circular. The process
of navigating through a network database was called “walking the set” because it involved
choosing paths through the database structure much like choosing walking paths through a
forest when there can be multiple ways to get to the same destination.
