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531 > ER Modeling > Flashcards

ER Modeling Flashcards

1
Q

Miniworld

A

part of the real world that we are interested in modeling

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2
Q

What is the importance of conceptual design?

A
  • Fundamental
  • Involves users
  • Provides good documentation for the Db
  • Independent of the DBMS
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3
Q

What is the Entity-Relationship (ER) model?

A

Notion that a database can be modelled as a collection of entity classes and relationships among them

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4
Q

What is the goal of the ER model?

A

Develop an ER schema or ER diagram

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5
Q

Entity

A

Data object that exists and is distinguishable from other objects

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6
Q

What can be considered entities?

A

An object that physically exists, abstract or organizational entity, or an event

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7
Q

Entity type

A

description of the attributes that a set of entities has in common

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8
Q

Entity set

A

A collection of of entities of the same type

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9
Q

Entity class

A

combination of entity set + entity type

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10
Q

Attribute

A

Property of an entity

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11
Q

All members of an entity class will have the same ______

A

Attributes
But they may have different attribute values
(e.g. all customers will have a name but the name will likely be different)

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12
Q

How is an entity class represented?

A

A rectangle

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13
Q

How is an attribute represented?

A

labelled oval connected to a entity class

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14
Q

How are entities distinguished from one another?

A

Each entity will have a unique combination of attribute values

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15
Q

Candidate Identifier

A

a set of one or more attributes whose values uniquely determine each entity.

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16
Q

Can you have more than one candidate identifier?

A

Yes there can be several

Ex: Name, DOB of customers, customerid of customers

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17
Q

Can you have more than one primary identifier?

A

No. One candidate identifier is selected to be the primary identifier

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18
Q

What conditions must be met for a primary identifier?

A
  • Unique
  • Mandatory
  • Time invariant
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19
Q

How are primary identifier represented?

A

Labelled oval with an underline

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20
Q

How are null values represented? When are they used?

A

@ symbol

For unknown or inapplicable values

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21
Q

How are multi-valued attributes represented? What is an example?

A

Double oval

Customer could have more than 1 phone number

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22
Q

What is a composite attribute? What is the opposite of a composite attribute?

A

A composite attribute is comprised of multiple values to describe it (e.g. address has street, city, zip etc.)
The opposite of a composite is a simple

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23
Q

What is a derived attribute? How is it represented? What is the opposite of a derived attribute?

A

The opposite of derived is stored. It is an attribute that is calculated from another attribute. It’s represented with a dotted oval. An example being age calculated from date of birth attribute

24
Q

Where are characteristics captured that are not typically represented in the ER diagram? What are some examples?

A

Data dictionary

  • Attribute cardinality
  • Entity class cardinality
  • Attributes that are optional or mandatory, attribute uniqueness
  • Domain constraints (e.g SSN must be 9 exactly)
25
Q

Cardinality

A

Number of elements in that set

26
Q

Cardinality constraint

A

bounds the upper and/or lower limits of the cardinality

27
Q

What does a cardinality constraint do for an attribute?

A

Restricts the number of values an attribute can take on

28
Q

Interaction relationship

A

association between or among entities

29
Q

Relationship type

A

is a list (ordered) of participating entity classes

30
Q

Relationship set

A

specific set of associated entity instances based on relationship type

31
Q

Relationship Class

A

relationship type + relationship set

32
Q

How is a relationship class represented?

A

Diamond

33
Q

Can a relationship class have attributes?

A

No

34
Q

How is data that exists due to the relationship classified as?

A

Weak entity

The data has no meaningful existence without the relationship that defines it (e.g order details)

35
Q

What is the purpose of a weak entity?

A

Provide more data about the interaction between the 2 entities

36
Q

Degree of an interaction relationship

A

Number of entity classes involved in a relationship

37
Q

What are examples of interaction relationships?

A

unary
binary
ternary

38
Q

Business Rule

A

business policy that leads to a rule/statement constraining some aspect of the business

39
Q

Where are Business rules and other or behavior-oriented/process oriented rules located?

A

Annotations on the ER or data dictionary

40
Q

Weak entity class

A

inherits identifying attributes from another entity class to uniquely identify its own entities

41
Q

How is a weak entity class represented?

A

Double lined box and an arrow pointing to the identifying relationship

42
Q

What is the defining characteristic of a weak entity?

A

If an entity class depends on another class for all or part of its identifier, it’s weak

43
Q

When does a weak entity have a partial identifier?

A

If the max cardinality > 1 in its identifying relationship

44
Q

How is a partial identifier represented?

A

Dotted underline

45
Q

What is the circle relationship symbol?

A

Indicates a superclass and associated subclass

46
Q

Designation

A

Classifying attribute for the superclass/subclass relationship

47
Q

What are the 4 types of superclass/subclass relationships

A

Disjoint, 0:1
Partition, 1:1
Overlap, 0:M
Cover, 1: M

48
Q

Disjoint superclass/subclass relationship

A

0:1 relationship
Example: an employee may not be an associate or a manager (just in the general employee pool)

49
Q

Partition

A

1:1
Example: Employee must either be a manager or an associate.
No employee can be both

50
Q

Overlap

A

0:M

An employee can be both a manager and an engineer (or could be neither)

51
Q

Cover

A

1:M

Employee has to be either a manager or engineer or could be both (can’t be neither)

52
Q

When to create subclasses

A
  • Attributes relative only to a subclass
  • Relationship classes relevant only to a subclass
  • Cardinality of relationships associated with subclasses are different
53
Q

How do subclasses gain attributes and relationships?

A

They are inherited from the superclass (including the identifier)

54
Q

Specific attributes

A

attributes to the subclass only

55
Q

Specific relationships

A

entities in a subclass participating in direct relationships

531 (4 decks)

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