Databases Illuminated, 4th Edition

3.7 Existence Dependency and Weak Entities

At times we need to store data about an entity that would not be in the database unless we had a related entity already in the database. For example, we would not need to store data about a sales order unless we had a customer. An existence constraint, or existence dependency, can occur between two entities. If X and Y are entities and each instance of Y must have a corresponding instance of X, then we say that Y is existence dependent on X. This means a Y entity cannot exist without some X entity.

If Y is existence dependent on X, then Y must have total participation in its relationship set with X. A special type of existence dependency occurs when the dependent entity set does not have a primary ...