quiz 1

Question 1

what are constraints in entity-relationship model

Answer 1

something that restricts relationships

Question 2

what is a composite attribute

Answer 2

attributes that can be divided into sub-parts, use sub-parts in diagram

Question 3

what are multivalued attributes

Answer 3

a single entity with multiple attrictbutes of the same type, not allowed for class, replace with a new entity set

Question 4

what are derived attributes and how are they denoted

Answer 4

attributes whos value can be determined by other values available. denoted with broken oval around attribute

Question 5

what is a key

Answer 5

a set of attributes whos values can uniquely identify an entity in an entity set

Question 6

what is a superkey

Answer 6

a set of attributes that can uniquely identify an entity in an entity set, a set can have multiple superkeys if multiple attributes and their values together can be unique

Question 7

what is a candidate key

Answer 7

a candidate key is a minimal superkey, no extrenuous or unnecessary attributes

Question 8

what is a primary key and how are they denoted

Answer 8

candidate key chosen to refer to rows in a table. table can only have one primary key. denoted with primary key attribute(s) underlined. a primary key is chosen because their attributes rarely/never change

Question 9

what is a relationship and its difference between relationship set

Answer 9

relationship is the association between entities. relationship sets are a set of relations of the same type

Question 10

true or false: relationship sets can have (descriptive) attributes

Answer 10

true, used for properties that cant be associated with either connected entity sets

Question 11

true or false: binary relationships can look like loops and if so will contain role descriptors

Answer 11

true slide 33

Question 12

what is mapping cardinalities

Answer 12

to specify how many entities of set A is associated with ONE entity in set B and vice versa

Question 13

what is a key constraint

Answer 13

A key constraint exists between set A and relationship R if entities in set A can only be associated with a single B entity (one to either one or many)

Question 14

what is a one to one relationship and how do you show it in a diagram

Answer 14

when an entity in A can only be associated with at more one entity in B AND an entity in B can only be associated with at most one entity in A. denoted with arrows from both entity set to relationship

Question 15

what is a one to many/many to one relationship and how to you denote it on a diagram

Answer 15

when an entity in A can be associated with any number of entities in B BUT an entity in B can be associated with at most one entity in A. denoted with the restricted set given an arrow and the whore set given a regular line

Question 16

what is a many to many relationship and how are they denoted

Answer 16

an entity in A can map to any number of entities in B AND an entity in B can map to any number of entities in A. denoted with both entity sets connecting to the relationship with a line. no arrows.

Question 17

what are descriptive attributes

Answer 17

attributes of a relationship set that cannot be apart of the primary key. primary key attributes are inherited from their connecting entity sets

Question 18

where are primary key attributes for a relationship from

Answer 18

derived from one or more of its entity sets, primary keys of relationship sets cannot include descriptive attributes that belong to the set

Question 19

describe many-to-many, one to many/many-to-one and one to one primary keys in relationship sets

Answer 19

many-to-many: relationship set’s primary key will be a compound of the primary keys from participating entity sets

one-to-many/many-to-one: the relationship set will take the same as the primary key from the entity with the key constraint (arrow)

one-to-one: the relationship set will take either primary key

Question 20

what is participation constraints, what are the two types and how do you denote them

Answer 20

participation constraints are used to indicate that an entity in an entity set must be involved in at least one relationship.

total participation (double line or thick line from relationship to entity): constraint exists, ie all must have a match in the other entity set

partial participation (regular line) : no constraint

Question 21

what is a weak entity set

Answer 21

where entities in an entity set cannot be uniquely identified without information from a related entity set

Question 22

what are subclasses

Answer 22

class hierarchies of entities

Question 23

what is aggregation

Answer 23

used to model the relationship/association that exists between entities and relationships

Question 24

how are entities of a weak entity set identifies and when are we permitted to using a weak set. how are weak sets denoted

Answer 24

identified by combining its partial key the primary key of another entity set

only allowed when the owner and weak set are in a one to many relationship and the weak set has total participation in the full entity set.

denoted with double like around the relationship between owner and weak set and a dotted line under the weak relationships partial key

Question 25

true or false: an entity of a subclass is an entity of the super class

Answer 25

true

Question 26

true or false: subclasses can have additional attributes outside of the super class

Answer 26

true, but all attributes of parents sets are inherited by subclasses along with the participation in relationship sets

Question 27

define specialization and generalization in subclasses

Answer 27

specialization: identify subsets with additional attributed from an existing entity set

generalization: identify common attributes of entity sets

Question 28

how to denote class hierarchy in the diagram

Answer 28

triangle

Question 29

define disjoint and overlap subclasses

Answer 29

disjoint: entities in one subclass cannot appear in another subclass

overlap: entities in one subclass may appear in another subclass

Question 30

what is a coverage constraint and how are the denoted on a digram

Answer 30

coverage constraint indicated that all superclass entity must belong to a subclass

denoted with a double line from the superclass to the triangle

Question 31

what is aggregation and when are they used and how do you denote them

Answer 31

indicated that a relationship set participates in a relationship with another relationship set

used when there is a relationship between an entity set and another relationship, often used with non-binary relationships

denoted with a rectangle around the relationship and entities that make the aggregate entity