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91.3902 Database Systems
Assignment #1

1. Discuss the role of a high-level data model in the database design process. High-level data models assist in conceptual design and helps express data requirements of the users and includes detailed descriptions of the entity types, relationships, and constraints. The high-level data model is also used as a reference to ensure that all users’ data requirements are met and that the requirements do not include conflicts.

2. List the various cases where use of a null value would be appropriate. Null value would be appropriate when a particular entity does not have an applicable value for an attribute. There are two cases for such situations. The first case arises when it is known that the attribute value exists but is missing. An example for cases like such would be if the Height attribute of a person is listed as null. The second case arises when it is not known whether the attribute value exists. An example would be is the HomePhone attribute of a person is null.

3. Define the following terms: entity, attribute, attribute value, relationship instance, composite attribute, multivalued attribute, derived attribute, complex attribute, key attribute, value set (domain).

Entity
Entity is a “thing” in the real world with an independent existence. It can also be an object with physical existence (i.e. person, car) or conceptual existence (i.e. company, job).

Attribute
An attribute is a particular property that describes entity (i.e. person name company name).

Attribute Value
Attribute values are major data stored in the database.

Relationship Instance
Relationship Instance is an association of entities, where the association includes exactly one entity from each participating entity type.

Composite Attribute
Composite attribute is an attribute that can be divided into meaningful components.

Multivalued Attribute
Multivalued attribute is an attribute that can have many values.

Derived Attribute
Derived attribute is an attribute whose value is computed from another attribute or combination of attributes.

Complex Attribute
Complex attribute is composite and multivalued attributes nested in an arbitrary manner.

Key Attribute
Key attribute is an attribute whose values are distinct for each individual entity in the collection.

Value Set (Domain)
Value set specifies the set of values that may be assigned to that attribute for each individual entry.

4. What is an entity type? What is an entity set? Explain the differences among an entity, an entity type, and entity set. Entity type defines a collection of entities that have the same attributes. Entity set is a named collection of related data. The differences between entity, an entity type, and entity set are: 1. Entity is a person, place, thing, event, or even a concept. It may be tangible or intangible. Examples of an entity are employees, 2. Entity type describes the schema or intension for a set of entities that share the same structure. 3. Entity set is the extension of the entity type.

5. Explain the difference between an attribute and a value set. An attribute is a particular property that describes entity. A value set specifies the set of values that may be assigned to that attribute for each individual entry. An example of an attribute is EmployeeAge and we can set the value set for this attribute to a range of integer numbers.

6. What is a relationship type? Explain the differences among a relationship instance, a relationship type, and a relationship set. Relationship type is the nature of a relationship between entities, expressed by the number of their possible occurrences in the related tables. The differences between a relationship instance, a relationship type, and a...
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