Other occurrences of the social security number attributes would be inherited across relations. Rules enforce creating proper models. In a situation where it seems that a rule cannot hold, the model is likely wrong. Keys A key is a group of attributes that uniquely identify an entity instance. There are primary and alternate keys.
Every entity has exactly one primary key displayed above the horizontal line in the entity box.
Entities can have alternate keys that also uniquely identify the entity, but are not used for describing relationships with other entities. In a connection relationship, the primary key of the parent migrates to the child. If the relationship is a category relation, the primary key of the child is the same as the generic. If the relationship is an identifying relationship, the primary key of the child must contain attributes inherited from the parent. Thus, when deciding whether an inherited attribute should be part of a key, an issue is whether that attribute is necessary for unique identification.