Week 5

Question 1

What are entity supertype and subtypes?

Answer 1

A subtype entity inherits the properties of the supertype entity (attributes and relationships) from the supertype entity. The subtype contains the unique attributes.

Question 2

How are relationships between subtypes and supertypes described?

Answer 2

Relationships are described in terms of “is-a” relationships

Question 3

What are some “key” things about inheritance?

Answer 3

• All entity subtype subtypes inherit attributes and relationships of the supertype
• All entity subtypes inherit their primary key attribute from their supertype
• At the implementation level, supertype an its subtypes maintain a 1:1 relationship
• Subtypes inherit all relationships in which supertype participates
• Lower level subtypes inherit all attributes and relationships from its all upper level supertypes

Question 4

What is a specialization hierarchy?

Answer 4

A specialization hierarchy depicts the parent-child relationship between a supertype and subtype.

Question 5

What is an ERR model?

Answer 5

An extended entity relationship model (EER) is a type of ERD that presents the specialization hierarchy.

Question 6

What are disjoint subtypes and overlapping subtypes?

Answer 6

• Disjoint Subtypes
  Subtypes that contain unique subset of supertype entity set
• Overlapping Subtypes
  Subtypes that contain nonunique subsets of supertype entity set

Question 7

What is partial completeness and total completeness?

Answer 7

• Partial completeness
  Not every supertype occurrence is a member of a subtype
• Total completeness
  Every supertype occurrence must be a member of any.

Question 8

What are the guidelines for a primary key?

Answer 8

A primary key should be:

• An attribute or combination of attributes that uniquely identifies entity instances in an entity set
• Main function is to uniquely identify an entity instance or row within a table
• To guarantee entity integrity, not to “describe” the entity

Question 9

What are some more guidelines for primary keys?

Answer 9

Primary keys should be:

• Non intelligent
• No change over time
• Preferably single-attribute
• Preferably numeric
• Security compliant

Question 10

When should use composite primary keys?

Answer 10

You should use composite primary keys when:

• There is a weak entity. Composite primary keys are normally used in this case to represent a real world object that is existent dependent on another real world object. Also used when a real world object that is represented in data model as two separate entities in strong identifying relationship

Question 11

What are surrogate keys?

Answer 11

A surrogate key is a unique identifier for an entity instance. Surrogate keys are used when there is:
- No natural key
- Selected candidate key has embedded semantic contents
- Selected candidate key is too long or cumbersome

Question 12

What is a natural key?

Answer 12

A natural key (or natural identifier) is a real world, generally accepted identifier used to uniquely identify real-world objects.