The Entity-Relationship Data Model

Other Kinds of Constraints

As already mentioned at the beginning of this section, there are other kinds of constraints one could wish to enforce in a database. We shall only touch briefly on these here, with the

Weak Entity Sets

There is a special condition in which an entity set's key is consisting of attributes some or all of which belong to another entity set. Such an entity set is called a weak entity set.

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.

Summary of The Entity-Relationship Data Model

The Entity/Relationship Model: In the E/R model we explain entity sets, relationships among entity sets, and attributes of entity sets and relationships. Members of entity sets are called entities.

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