Given the data in the two tables, you can see that an AGENT – through the AGENT_CODE – can occur many times on the CUSTOMER table. But each CUSTOMER has only one AGENT.
This covers part of the business rules as:
• Each AGENT can have many customers
• Each CUSTOMER may have one AGENT.
However, this is very imprecise. Words such as “can” or “may” imply that the relationship is
optional, causing the crow’s-foot connector on the associated ERD to appear as this:
This symbol is always overused by students because it is misrepresented in the text. Optional
relationships are much more infrequent than what the text leads the reader to believe.
If we look at the data more closely, we see that there is no CUSTOMER who is not assigned an
AGENT; nor is there any AGENT who is not assigned a CUSTOMER. Therefore, the relationship is
NOT OPTIONAL. The business rules should not be written without “can” or “may”, and the crow’sfoot connector on the associated ERD to appear as this:
• Each AGENT has many customers
• Each CUSTOMER has one AGENT.
In addition, it is impossible to jump right to “many” without having at least a zero or a one, first. So the
business rule must also include that starting point in the relationship description. So the final, precise
business rules are written as:
• Each AGENT is assigned one to many customers
• Each CUSTOMER is handled by one and only one AGENT.
Written this way, we account for both beginning and ending symbols in the relationship, as well as the
bi-directional nature of all relationships.