Week 9

Question 1

Relationship

Answer 1

Denotes a connection between entity classes.

Should be named with a verb phrase
one phrase - grammar modified to reflect each direction
two phrases - specific for each direction.

Question 2

relationship instance

Answer 2

denotes a connection between entity instances

Question 3

Degree

Answer 3

The degree of a relationship is the number of entity classes that participate in the relationship.

Question 4

Recursive

Answer 4

A relationship among entities of the same type.

Will always have a degree of 1.
Role names are helpful to determine perspective. (mentee, mentor)

Question 5

Binary

Answer 5

Exactly two entities. Degree of 2.

Question 6

Maximum Cardinality

Answer 6

the maximum number of entity instances that can occur on one side of a relationship
cardinality ratio: one-to-one, one-to-many or many to one, many to many
if a specific maximum is known you can use the number

Question 7

Determining maximum cardinality

Answer 7

can entity a have more than one entityb?
can entity b have more than one entity ?

one line for one, crows foot for many

Question 8

Determining Mimimum cardninality

Answer 8

Does entity_a have to have an entity_B? and vice versa.

If no, than min cardinality is zero. Enoted by an oval placed before the maximum cardinality.

If yes, denoted by a perpendicular line to the relationship, placed before the max cardinality for the entity that is required to participate in the relationship

Question 9

Supertype/subtype relatinship

Answer 9

IS_A
generalization hierachies

Question 10

Supertype

Answer 10

A type of entity class that contains attributes that are common accross all specializations

Question 11

Subtype

Answer 11

a type of entity class that “inherits” attributes of its supertype as well as containing attributes of its own

Question 12

Strong vs Weak entities

Answer 12

strong – can exist on its own
weak cannot (subtypes). Denoted by rounded corners or double outline

Question 13

supertype/subtype basic notation

Answer 13

circle with arrows,
subtypes don’t have identifiers

Question 14

when is a supertype/subtype necessary?

Answer 14

there are attributes that apply to some, but not all, instances of an entity
there is a relationship that pertains to a defined subset of instances of an entity

Question 15

Completeness Constraint

Answer 15

The minimum number of subtype instances that a supertype instance must belong to.
Must every instance of the supertype belong to a subtype?
§ If an answer is ‘NO, an instance of the supertype doesn’t have to belong to any of the subtype(s)’, then
there is partial specialization.
§ Denoted by a single line from the supertype to the supertype/subtype relationship indicator.
§ If an answer is ‘YES, every instance of the supertype must belong to a subtype’, then there is total
specialization.
§ Denoted by double lines from the supertype to the supertype/subtype relationship indicator.

Question 16

Disjointedness constraints

Answer 16

The maximum number of subtype instances that a supertype instance can belong to.
§ To determine the type of disjointedness constraint, you must ask a question.
§ Can an instance of the supertype belong to more than one subtype?
§ If an answer is ‘NO, an instance of the supertype may only belong to at most one subtype’, then the
disjoint rule is in effect.
§ Denoted by a ‘d’ inside the supertype/subtype relationship indicator.
§ If an answer is ‘YES, an instance of the supertype can belong to more than one subtype’, then the
overlap rule is in effect.
§ Denoted by an ‘o’ inside the supertype/subtype relationship indicator.

Question 17

Subtype Discriminator

Answer 17

A discriminator is an attribute, or a group of attributes, in a supertype that will indicate any subtypes that an entity instance belongs to.
Disjoint Rule
§ A single attribute can be used as a discriminator in the supertype.
§ The value for the discriminator will indicate which subtype, if any, an entity instance of the
supertype belongs to.
§ Overlap Rule
For each subtype an attribute will need to be used as a discriminator in the supertype.
The value of each attribute will indicate which subtypes an entity instance belongs to.

Question 18

Four options for supertype/subtypes?

Answer 18

Total specialization and disjoint rule: Every instance of the supertype must belong to one and only one subtype.

Total specialization and overlap rule: Every instance of the supertype must belong to at least one subtype, or any combination ofsubtypes.

Partial specialization and disjoint rule: An instance of the supertype does not have to belong to a subtype, but if it does, it will belong to only one subtype.

Partial specialization and overlap rule: An instance of the supertype does not have to belong to a subtype or it may belong to any combination of subtypes.