Many-one Relationship

Multiplicity of Binary E/R Relationships

Generally, a binary relationship can connect any member of one of its entity sets to any number of members of the other entity set. On the other hand, it is common for there to be a restriction on the "multiplicity" of a relationship. Assume R is a relationship connecting entity

Multiway Relationships

The E/R model makes it easy to describe relationships involving more than two entity sets. In practice, ternary (three-way) or higher-degree relationships are infrequent, but they are rarely necessary to reflect the true state of affairs. A multiway relationship in an E/R diagram

Simplicity Counts

Avoid introducing more elements into your design than is really needed.

Referential Integrity

While single-value constraints show that at most one value exists in a given role, a referential integrity constraint asserts that exactly one value exists in that role. We could see

Requirements for Weak Entity Sets

We cannot get key attributes for a weak entity set indiscriminately. Rather, if E is a weak entity set then its key comprises:

Weak Entity Set Notation

We shall adopt the following principles to show that an entity set is weak and to declare its key attributes.

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