OBJECT-BASED DATA MODEL: PRINCIPLES AND COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 199

1

PERSON Is married to

1

1

WORKER manages

∗

Figure 6-26 Recursive relationships.

*

ITEM contains

*

Quantity

Figure 6-27 Assembly structure.

of association is a recursive relationship. Your data model must be able to indicate
recursive relationships.
Figure 6-26 shows examples of recursive relationships.
Note how the relationship line from an object is linked back to itself. Cardinali-
ties may vary with recursive relationships just like they do with regular relation-
ships. Observe the different sets of cardinality indicators represented in the figure.

Assembly Structures

What is a bill of materials structure? You come across bill of materials processing
(BOMP) in manufacturing. In manufacturing of automobiles, an assembly of a
major part consists of several subassemblies; each subassembly in turn may be
broken down further. Assembly structures are cases of recursive relationships in
which the associations recur at many levels.
Figure 6-27 displays an example of an assembly structure.
Notice the attribute Quantity shown in the diagram for the object ITEM. This
attribute Quantity does not represent quantities of individual instances of the
object. Quantity indicates units for the various recursive associations. How many
units are there for the combinations of item number 1 with item numbers 4,5, and
6? It is an attribute of an aggregate object consisting of just one object ITEM.