Primary keys provide a way to specify how entities and relationships are distinguished.
Primary key for entity sets
By definition, individual entities are distinct. However, from a database perspective, the differences among them must be expressed in terms of their attributes.
Therefore, the values of the attribute values of an entity must be such that theycan uniquely identify the entity.
The concepts of superkey, candidate key, and primary key are applicable to entity sets just as they are applicable to relation schemas.
Primary key for relationship sets
To distinguish among the various relationships of a relationship set, the individual primary keys of the entities in the relationship set are used.
Let be a relationship set involving entity sets . Let be the set of attributes assoiciated with , which may be . Then, the set of attributes
forms a primary key for .
The choice of the primary key for a relationship set depends on the mapping cardinality of the relationship set.
Weak entity sets
In some cases, the primary key of an entity in a relationship is redundant when describing the relationship. In this case, an alternative way to deal with this redundancy is to not store the redundant attribute in the entity.
A weak entity is one whose existence is dependent on another entity, called its identifying entity. An entity set that is not a weak entity set is termed a strong entity set.
Instead of associating a primary key with a weak entity, we use the identifying entity, along with extra attributes called a discriminator or partial key to uniquely identify a weak entity.
Every weak entity must be associated with an identifying entity; the weak entity set is said to be existence dependent on the identifying entity set.
- The identifying entity set is said to own the weak entity set that it identifies.
- The relationship associating the weak entity set with the identifying entity set is called the identifying relationship.