Weak Entity Types

Weak entity types

Entity types that do not have key attributes of their own are called weak entity types like the DEPENDENT entity type in the company database.

In contrast, regular entity types that do have a key attribute are called strong entity types. Entities belonging to a weak entity type are identified by being related to specific entities from another entity type in combination with one of their attribute values. We call this other entity type the identifying or owner entity type, and we call the relationship type that relates a weak entity type to its owner the identifying relationship of the weak entity type.

Consider the entity type DEPENDENT, related to EMPLOYEE, which is used to keep track of the dependents of each employee via a 1:N relationship. In the diagram below, the attributes of DEPENDENT are Name, Birth_Date, and Relationship (to the employee). Two dependents of two distinct employees may, by chance, have the same values for Name, Birth_Date, and Relationship, but they are still distinct entities. They are identified as distinct entities only after determining the particular EMPLOYEE entity to which each dependent is related.

In ER diagrams, both a weak entity type and its identifying relationship are distinguished by surrounding their boxes and diamonds with double lines.