Chapter 7: Problem 1

Discuss the correspondences between the ER model constructs and the relational model constructs. Show how each ER model construct can be mapped to the rela. tional model, and discuss any alternative mappings.

Short Answer

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In the ER model, entities map to tables in the relational model, and attributes map to columns. Relationships in the ER model can be translated into separate tables in the relational model by joining primary keys of the associated entities. Complex attributes are usually represented as separate tables. Alternative mappings might include using null values to represent optional relationships or moving relationship attributes to one of the related entities to simplify the model.

Step by step solution

Understanding ER and Relational Models

The ER model represents the conceptual design of database and includes entities ('things' about which data is being captured), attributes (properties of entities) and relationships (associations between entities). Conversely, the relational model represents the logical structure of the database, using tables (or relations) to represent data, and these tables are linked through primary and foreign keys.

Mapping Entities to Tables

As the first step of mapping, each entity in the ER model is converted into a table in the relational model. The attributes of the entity in the ER model are transformed into columns of the table in the relational model. An additional column for unique identification, known as the Primary Key, is also added.

Mapping Relationships to Tables

Next, each relationship in the ER model is mapped to a table in the relational model. To facilitate this, the primary keys from each related entity are combined to form a new table in the relational model. Each row in the relationship table represents an association between the two entities. Additionally, any attributes of the relationship are also added as columns in this table.

Mapping Complex Attributes

Complex attributes, or attributes composed of other attributes, are mapped by creating a separate table for the complex attribute. The primary key of the original entity is used as the primary key of the new table.

Discussing Alternative Mappings

There may be cases where alternative mappings are possible. For instance, in case of optional relationships where entities may exist without a relationship, a null value could be used to represent absence of relationship rather than creating a separate table. Also, if an attribute of a relationship can be comfortably moved to one of the related entities, it could simplify the relational model.

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Discuss the correspondences between the ER model constructs and the relational model constructs. Show how each ER model construct can be mapped to the rela. tional model, and discuss any alternative mappings.

Short Answer

Step by step solution

Understanding ER and Relational Models

Mapping Entities to Tables

Mapping Relationships to Tables

Mapping Complex Attributes

Discussing Alternative Mappings

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