Database(15) -- The Entity/Relationship Model

Weak Entity Sets

If an entity set’s key is composed of attributes, some or all of which belong to another entity set, such an entity set is called a weak entity set.

Causes of Weak Entity Sets

If entities of set E are subunits of entities in set F, then it is possible that the names of E-entities axe not unique until we take into account the name of the F-entity to which the E entity is subordinate.

The second common source of weak entity sets is the connecting entity sets as a way to eliminate a multiway relationship.

Requirements for Weak Entity Sets

If E is a weak entity set then its key consists of:

1. Zero or more of its own attributes
2. Key attributes from entity sets that are reached by certain many-one relationships from E to other entity sets. These many-one relationships are called supporting relationships for E, and the entity sets reached from E are supporting entity sets.

Weak Entity Set Notation

1. If an entity set is weak, it will be shown as a rectangle with a double border.
2. Its supporting many-one relationships will be shown as diamonds with a double border.
3. If an entity set supplies any attributes for its own key, then those attributes will be underlined.