Database Reference
In-Depth Information
Figure 3-6.
Example of a Hierarchy (Tree)
Database Networks
The database network approach is another traditional approach that is no longer
employed. In the interest of historical context, a brief overview is provided here. A
simple
database network
is a collection of entities and 1:M relationships arranged such that
any member can have multiple parents, providing that the parents are different entities.
Figure
3-7
illustrates the approach. It was successfully employed in a DBMS called the
CODASYL system (the original CODASYL acronym stands for “Conference on Data
Systems Languages”).
Figure 3-7.
A Simple Network
A
complex database network
is a collection of entities and relationships, at least
one of the relationships being an M:M relationship. Figure
3-8
illustrates. The complex
network can be reduced to a simple network by replacing all M:M relationships with M:1
relationships. The technique for replacing M:M relationships will be discussed in the
upcoming subsection.
