The attributes of a relationship set are the attributes that comprise the
primary keys of the entity sets involved in the relationship set.
For example:
- S.I.N. is the primary key of customer, and
- account-number is the primary key of account.
- The attributes of the relationship set custacct are then
(account-number, S.I.N.).
This is enough information to enable us to relate an account to a person.
If the relationship has descriptive attributes, those are also included in
its attribute set. For example, we might add the attribute date> to the
above relationship set, signifying the date of last access to an account by a
particular customer.
Note that this attribute cannot instead be placed in either entity set as it
relates to both a customer and an account, and the relationship is many-to-many.
The primary key of a relationship set
depends on the mapping cardinality and the presence of descriptive attributes.
With no descriptive attributes:
- many-to-many: all attributes in
.
- one-to-many: primary key for the "many" entity.
Descriptive attributes may be added, depending on the mapping cardinality and
the semantics involved (see text).
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