1.1.9 Relationship Guidelines

Rules for relationships are often imposed by modeling tools. Following are some general relationship guidelines:

• An entity cannot have a parent relationship with itself, nor can it have as a parent, any entity to which it is directly or indirectly a parent.

• An entity cannot have an entity as a parent if it already has one of that entity's children as a parent. Such a relationship would be superfluous. It can, however, join to such an entity.

• An entity may have more than one parent relationship with another entity.

• An entity may have a join relationship with any other entity, including itself.

• An entity may have more than one join relationship with another entity.

• An entity may have both a parent and join relationship with another entity.

Note: The relationship rules or guidelines enforced will depend upon the data modeling tool used.