Week 2 Revision

Question 1

What is inheritance?

Answer 1

The process by which a lower level entity set inherits all the attributes and relationship participation of the higher level entity set to whcih it is linked

Question 2

What are some benefits of inheritance?

Answer 2

<ul>
<li>Avoids duplication of data</li>
<li>Allows relationships between similar types of entities to be represented</li>
</ul>

Question 3

What does the completeness constraint specify?

Answer 3

Whether an entity in the superclass entity set must belong to at least one of the entity sets within a generalization

Question 4

What are the two types of constraint inheritance?

Answer 4

Total

Partial

Question 5

Describe total inheritance

Answer 5

An entity MUST belong to one of the subclass entity sets

Question 6

Describe partial inheritance

Answer 6

An entity need not belong to one of the subclass entity sets, can be a member of the superclass

Question 7

What does the inheritance constraint specify?

Answer 7

Constrinat on whether or not entities may belong to more than one subclass entity set within a single generalization

Question 8

What are the two types of inheritance?

Answer 8

Overlapping

Disjoint

Question 9

What is overlapping inheritance?

Answer 9

An entity can belong to more than one subclass

Question 10

What is disjoint inheritance?

Answer 10

An entity can belong to at most one suclass

Question 11

What are some ER design decisions that need to be made at the modelling stage?

Answer 11

<ul>
<li>Entity set vs attribute</li>
<li>Etity set vs relationship set</li>
<li>Placement of attribute on entity set or relationship set</li>
<li>Use of n-ary relationship set vs multiple binary</li>
<li>Strong vs weak entity set</li>
<li>Specalisation</li>
</ul>

Question 12

When would you use an entity set over an attribute?

Answer 12

When you want/need to store additional information aboyt that field i.e. mobile number

Question 13

When would you use a relationship set over an entity set?

Answer 13

If all the relationships leading from an entity set are complete (have to participate) and are described using generic verbs such as ‘has’

Question 14

Where is an attribute of a many to many relationship placed?

Answer 14

On the relationship set

Question 15

Where is an attribute of a one to many relationship set placed?

Answer 15

On the relationship set or the many side entity set

Question 16

Where is an attribute of a one to one relationship set placed?

Answer 16

On the relationship set or in either entity set

Question 17

Even though it’s possible, what is a reason for not replacing any n-ary relationship set with binary relationship sets?

Answer 17

An n-ary relationship set shows more clearly that several entities participate in a single relationship

Question 18

What are some alternatives to subclasses?

Answer 18

<ul>
<li>Use an attribute to distinguish between groups</li>
<li>Use relationships and roles to distinguish between groups</li>
</ul>

Question 19

When could you use an attribute instead of subclasses?

Answer 19

When subclasses have no attributes or relationships (fiction vs non fiction)

Question 20

When could you use roles instead of subclasses?

Answer 20

<ul>
<li>Potential subclasses have the same attributes</li>
<li>Specalisation would be overlapping</li>
<li>Relationships with potential subclasses are optional</li>
</ul>

Question 21

What are the four steps in the simple modelling methodology?

Answer 21

<ol>
<li>Decide what entity sets are required</li>
<li>For each entity set, decide on attributes</li>
<li>Decide on relationships between entity sets</li>
<li>Decide on generalisation/specialisation</li>
</ol>

Question 22

What does cardinality deal with?

Answer 22

Maximum times they can participate

Question 23

What does participation deal with?

Answer 23

Minimum times they can participate

Question 24

Describe the relational model

Answer 24

<ul>
<li>One step down from the E.R model</li>
<li>A means of storing information in tables called relations</li>
<li>Defines the structure, manipulation and constraints</li>
</ul>

Question 25

What is the domain of an attribute?

Answer 25

The set of allowed values for each attribute

Question 26

What does atomic mean in the context of attribute values?

Answer 26

Indivisible, cannot be chopped into any smaller chunks

Question 27

What are some properties of relations?

Answer 27

• Each relation has a distinct name
• Each cell is atomic
• Attributes in a relation have distinct names
• Values of an attribute are of the same domain
• There are no duplicate tuples in a relation
• The order of the attributes and the tuples has no significance

Question 28

What is the definition of a superkey?

Answer 28

Let K be a set of attributes in the relation schema R. K is a superkey of R if values for K are sufficient to identify a unique tuple of each possible relation r(R).

Question 29

What makes a superkey a candidate key?

Answer 29

A superkey K is a candidate key if K is minimal.

Question 30

How is a primary key represented in relational schema?

Answer 30

Underlined

Question 31

How is a foreign key represented in relational schema?

Answer 31

Followed by a star

Question 32

What are the two pure relational query languages?

Answer 32

1. Relational algebra
2. Relational calculus

Question 33

What type of language is relational algebra?

Answer 33

Procedural

Question 34

What type of language is relational calculus?

Answer 34

Declarative

Question 35

Describe relational algebra

Answer 35

Relational algebra explains how to obtain the result, by giving a functional description in terms of mathematical functions

Question 36

Describe relational calculus

Answer 36

Relational calculus explains what is required to obtain the result, by giving a declarative description

Question 37

What are relations in relational algebra?

Answer 37

Operands

Question 38

What are the various operations such as select and join in relational algebra?

Answer 38

Operators

Question 39

What are the two types of operations in relational algebra?

Answer 39

Unary and binary

Question 40

What are the unary operations?

Answer 40

Select, project, rename

Question 41

What are the binary operations?

Answer 41

Union, set difference, cartesian product

Question 42

What are the derived operations of relational algebra?

Answer 42

Set intersection, natural join

Question 43

Describe the select operation

Answer 43

Equivalent to filtering, will only return the rows where a certain condition holds (the selection predicate)

Question 44

Describe the project operation

Answer 44

Will return the columns that are listed in the project operation and nothing else

Question 45

Describe the rename operation

Answer 45

Will allow reference t a relation by more than one name

Question 46

Describe the union operation

Answer 46

Will return tuples that appear in both of the relations

Question 47

Define arity

Answer 47

Number of attributes in a relation

Question 48

What must hold for union of two relations to be valid?

Answer 48

1. The two relations must have the same arity
2. The second column of each table must have the same type of value as the other table

Question 49

Describe the set difference operation

Answer 49

Will return tuples that appear in the first but not the second relation

Question 50

What must hold for the set difference of two relations to be valid?

Answer 50

1. The two relations must have the same arity
2. The attribute domains of the relations must be compatible

Question 51

Describe the cartesian product operation

Answer 51

Will return each tuple in one relation joined with each tuple in the other relation

Question 52

Describe the set intersection operation

Answer 52

Will return tuples that appear in both of the given relations

Question 53

What must hold for the set intersection operation to be valid?

Answer 53

Relations must be compatible

Question 54

What does it mean to be compatible relations?

Answer 54

1. Relations must have the same arity
2. Attribute domains of the two relations must be compatabile

Question 55

Describe the natural join operation loosely

Answer 55

Combine relations with matching values of common attributes in both relations

Question 56

Describe the natural join operation in terms of other operations

Answer 56

1. Form cartesian product of two relations r and s 2. Perform a selection using equality on all common attributes 3. Remove duplicate attributes using projection

Question 57

Describe a relation

Answer 57

Set of tuples whcih represent instances of an entity set

Question 58

Describe a relational schema

Answer 58

Blueprint used to describe what information will be stored in a database and how it'll be divided into relations

Question 59

List two reasons why null values may introduced into a database

Answer 59

1. Indicate information is missing
2. Indicate information is unknown