Recursive Relationships
Recursive Relationships
An entity can participate in a recursive relationship (also called “fishhook”) in which the same entity is both the parent and the child. This relationship is an important one when modeling data originally stored in legacy DBMSs such as IMS or IDMS that use recursive relationships to implement bill of materials structures.
For example, a COMPANY can be the “parent of” other COMPANYs. As with all non-identifying relationships, the key of the parent entity appears in the data area of the child entity.
Recursive Relationship Example
The recursive relationship for COMPANY includes the diamond symbol to indicate that the foreign key can be NULL, such as when a COMPANY has no parent. Recursive relationships must be both optional (diamond) and nonidentifying.
The “company-id” attribute is migrated through the recursive relationship, and appears in the example with the rolename “parent-id.” There are two reasons for this. One, as a general design rule, an attribute cannot appear twice in the same entity under the same name. Thus, to complete a recursive relationship, you must provide a rolename for the migrated attribute.
Two, the attribute “company-id” in the key, which identifies each instance of COMPANY, is not the same thing as the “company-id” migrated through the relationship, which identifies the parent COMPANY. You cannot use the same definition for both attributes, so the migrated attribute must be rolenamed. An example of possible definitions appears next:
company-id: |
The unique identifier of a COMPANY. |
parent-id: |
The “company-id” of the parent COMPANY. Not all |
|
COMPANYs have a parent COMPANY. |
Refining Model Relationships 6–15
Recursive Relationships
If you create a sample instance table, such as the one following, you can test the rules in the relationship to ensure that they are valid.
COMPANY
company-id |
parent-id |
company-name |
|
|
|
C1 |
NULL |
Big Monster Company |
|
|
|
C2 |
C1 |
Smaller Monster Company |
|
|
|
C3 |
C1 |
Other Smaller Company |
|
|
|
C4 |
C2 |
Big Subsidiary |
|
|
|
C5 |
C2 |
Small Subsidiary |
|
|
|
C6 |
NULL |
Independent Company |
|
|
|
Sample Instance Table for COMPANY
The sample instance table shows that “Big Monster Company” is the parent of “Smaller Monster Company” and “Other Smaller Company.” “Smaller Monster Company,” in turn, is the parent of “Big Subsidiary” and “Small Subsidiary.” “Independent Company” is not the parent of any other company and has no parent. “Big Monster Company” also has no parent. If you diagram this information hierarchically, you can validate the information in the table.
COMPANY Hierarchy
6–16 ERwin Methods Guide