Session 12 (E/R Diagrams)

Question 1

what kinds of questions do ER diagrams answer?

Answer 1

 What is data?
 How are the data related to each other?
 How can we query the data and change it?
 How do we ensure that the data represents a
correct view of the real world (data integrity)?

Question 2

what is an entity (entity classes?)

Answer 2

Person, place, object, event or concept about

which data is maintained

Question 3

Named property of an entity is an ___

Answer 3

attribute (info or field)

Question 4

what are relationships?

Answer 4

Ties between different entities

Question 5

give an ERD example

Answer 5

Entities (Entity Classes)
 e.g., Student, Course, Patient, Doctor
 Attribute (Information or field)
 e.g., Student ID, Course Name, Credits, Medical
School
 Relationships
 e.g., Student takes a class

Question 6

what is a primary key

Answer 6

 Attribute(s) that can uniquely identify a given entity

(or relationship) instance.

Question 7

what is a foreign key

Answer 7

Attribute(s) that can uniquely identify a given entity

(or relationship) instance in another entity

Question 8

what is a composite attribute

Answer 8

An attribute that has meaningful component parts

attributes

Question 9

what is a multivalued attribute

Answer 9

 An attribute that may take on more than one value for a given entity (or relationship) instance.

Question 10

how is entity integrity maintained in a primary key?

Answer 10

Unique identifier of record
 Can’t be duplicate
 Can’t be null

Question 11

what is a candidate key?

Answer 11

Attribute (or combination of attributes) that

uniquely identifies each instance of an entity type

Question 12

what is an identifier (primary key?)

Answer 12

A candidate key that has been selected as the

unique identifying characteristic for an entity type

Question 13

describe cardinalities

Answer 13

express the number of entities to which another entity may be associated via a relationship
 Maximum cardinality
 Minimum cardinality

Question 14

what is a relationship

Answer 14

describes a physical or conceptual connection between entities. It is always bi-directional

Question 15

what are the three types of binary relationships?

Answer 15

1. one to one
2. one to many
3. many to many

Question 16

describe a one to one relationship

Answer 16

Maximum cardinality of one for both sides

Question 17

describe a one to many relationship

Answer 17

One entity with maximum cardinality equal to one

and one with maximum cardinality equal to N.

Question 18

describe a many to many relationship

Answer 18

Relationships with a maximum cardinality of N for

both entities