Attributes

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.

The Relational Data Model

While the entity-relationship approach to data modeling that we discussed in "The Entity-Relationship Data Model" is a simple and suitable way to explain the structure of data,

Attributes / Schemas

Across the top of a relation we see attributes; in the following figure, the attributes are title, year, length, and filmType. Attributes of a relation serve as names for the columns of the

Tuples / Domains

The rows of a relation, other than the header row containing the attribute names, are called tuples. A tuple has one component for each attribute of the relation. For example, the first of the three

Equivalent Representations of a Relation

Relations are sets of tuples, not lists of tuples. Therefore the order in which the tuples of a relation are presented is unimportant. For instance, we can list the three tuples of the

Relation Instances

A relation about movies is not stationary; rather, relations change over time. We expect that these changes include the tuples of the relation, such as insertion of new tuples as movies

From Entity Sets to Relations

We shall first consider entity sets that are not weak. We shall take up the modifications required to accommodate weak entity sets in "Handling Weak Entity Sets". For each

From E/R Relationships to Relations

Relationships in the E/R model are also represented by relations. The relation for a given relationship R has the following attributes:

Combining Relations

Often, the relations that we get from converting entity sets and relationships to relations are not the best possible choice of relations for the given data. One ordinary situation occurs when there

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