Analyzing Data › Defining Entity Types, Relationships, and Attributes › Defining Relationships › Number of Relationship Pairings

Number of Relationship Pairings

Another piece of information that can be specified is the estimated number of pairings participated in by an entity, that is the single member in a many relationship memberships. This information is useful during performance analysis and database design.

In the following example, there is only one many relationship memberships: customer places one or more orders. The number of pairings of interest represents the potential number of orders to be placed by a single customer.

In CA Gen, you specify the following information for the number of pairings:

• Minimum number of pairings for the single entity, and whether that minimum is estimated or absolute.
• For optional memberships, the minimum is obviously zero. However, the value that is specified here presupposes the existence of at least one pairing. So, consider "the minimum number of pairings if any exist."
• Maximum number of pairings for the single entity, and whether that maximum is estimated or absolute.
• Average number of pairings for the single entity

For example, assume that experience teaches the business the following things:

• Each CUSTOMER who has any ORDERs outstanding typically has at least two.
• A CUSTOMER is never expected to have more than ten outstanding ORDERs outstanding.
• Most CUSTOMERs have three outstanding ORDERs.

These numbers are estimates, and a customer who wishes to place an 11 order will not be turned away based on a technicality. Given these assumptions, the sentences is modified as follows:

• Each CUSTOMER sometimes (95 percent) places at least two (estimated), at most 10 (estimated) and on average three ORDERs
• Each ORDER always is placed by exactly one CUSTOMER

The second membership remains the same because it is neither optional nor many.

A relationship membership in which the minimum, maximum, and average are equal, and for which the minimum and maximum are absolute, is said to have fixed cardinality.

There are cases where the number of pairings that are allowed varies depending on combinations of values. For example, a customer whose credit rating indicates that a questionable risk is allowed one outstanding order at a time. A customer with a more favorable rating is allowed to place many orders at one time. You can document such a rare situation by recording a cardinality condition in the description of the affected relationship membership.