Other ER modeling tools

Other ER modeling tools include double ellipses and double lines. These tools permit the representation of other constraints, multivalued attributes, and the specification of full participation. In addition, it is possible for a relationship to have an attribute, that is, for a diamond to have attributes that are part of the relationship, not part of the entities. Useful references for more advanced ER modeling-and enhanced ER (EER) modeling-are provided in Appendix E.

Double lines as relationships indicate full participation and represent cases where an instance of one entity can't exist without a corresponding instance of the entity that it is related to. An example is an order as discussed in the previous section. An order can't exist without a customer to make that order. Therefore, correctly, the relationship between order and customer should be represented as a double line; the same constraints apply in the model to items and inventories.

Dashed ellipses represent multivalued attributes, attributes that may contain more than one instance. For example, the attribute address can be multivalued, because there could be a business address, a postal address, and a home address. Multivalued attributes aren't used in our model.

In addition, there are other extensions to the modeling techniques that have already been applied. For example, more than two entities can be related in a relationship (that is, more than two entities can be connected to a diamond). For example, the sale of a wine can be described as a three-way relationship between a wine, a customer, and an order. A second complex technique is the composite attribute; for example, an attribute of customer is address and the attribute address has its own attributes, a street, city, and zipcode. We don't explore complex relationships in this tutorial.