Discrete maths
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Discrete Maths. 242-213 , Semester 2, 2013 - 2014. Objective to introduce relations, show their connection to sets, and their use in databases. 5. Relations. Overview. Defining a Relation Relations on a Set Properties of a Relation reflexive, symmetric, transitive

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Discrete maths

Discrete Maths

242-213, Semester 2,2013-2014

  • Objective

    • to introduce relations, show their connection to sets, and their use in databases

5. Relations


Overview

Overview

  • Defining a Relation

  • Relations on a Set

  • Properties of a Relation

    • reflexive, symmetric, transitive

  • Combining Relations as Sets

  • Composition of Relations

  • N-ary Relations

  • Databases and Relations

  • More Information


1 defining a relation

1. Defining a Relation

  • We define a relation as the connection between two (or more) sets.

  • We need to first define:

    • the ordered pair

    • the Cartesian Product for two or more sets


An ordered pair

An Ordered Pair

  • In an ordered pair of elements a and b, denoted (a, b), the ordering of the entries matters.

  • So and iff a = cand b = d


The cartesian product of 2 sets

The Cartesian Product of 2 Sets

  • The Cartesian Product of sets A and B is the set consisting of all the ordered pairs (a, b),where :

  • One example Cartesian product is the Euclidean plane, which is the set of all ordered pairs of real numbers

  • In general


A definition of relation

A Definition of Relation

  • A relationR from a set A to a set B is any subset of the Cartesian product A x B.

  • Thus, if P = Ax B, , then we say that x is related to y by R and use the notation: xRy


Relation example

Relation Example

  • Let A={Smith, Johnson} be a set of students

  • B={Calc, Discrete Math, History, Programming} is a set of subjects.

  • Smith takes Calc and History, while Johnson takes Calc, Discrete Math and Programming.

  • Define relation R as: “student xtakes subject y”.

    • R={ (Smith, Calc), (Smith, History), (Johnson, Calc), (Johnson, Discrete Math), (Johnson, Programming) }

  • So Smith R Calc is true, whileJohnson RHistory is false.


2 relations on a set

2. Relations on a Set

  • A relation on the set A is a relation from A to A.

  • In other words, a relation on the set A is a subset of AA

  • Example: Let A = {1, 2, 3, 4}. Which ordered pairs are in the relation R = {(a, b) | a < b} ?

R = { (1,2), (1,3), (1,4), (2,3), (2,4), (3,4) }

continued


Graphically

Graphically:

R = { (1,2), (1,3), (1,4), (2,3), (2,4), (3,4) }

to

1

1

X

X

X

2

2

from

X

X

3

3

X

4

4


No of relations in a set

No. of Relations in a Set

  • How many different relations can we define on a set A with n elements?

  • A relation on a set A is a subset of AA.

  • How many elements are in AA?

  • There are n2 elements in AA, so how many subsets (== relations on A) does AA have?

  • The number of subsets that we can form out of a set with m elements is 2m. Therefore, 2n2 subsets can be formed out of AA.

  • Answer: We can define 2n2 different relations on A.


3 properties of a relation

3. Properties of a Relation

A relation R on a set Smay have special properties:

  • If , is true, then R is called reflexive.

  • If is true whenever is true, then R is called symmetric.

  • If is true whenever are both true, then R is called transitive.


Reflexive examples

Reflexive Examples

  • Are the following relations on {1, 2, 3, 4} reflexive?

R = {(1, 1), (1, 2), (2, 3), (3, 3), (4, 4)}

No.

R = {(1, 1), (2, 2), (2, 3), (3, 3), (4, 4)}

Yes.

R = {(1, 1), (2, 2), (3, 3)}

No.


Transitivity examples

Transitivity Examples

  • Are the following relations on {1, 2, 3, 4} transitive?

R = {(1, 1), (1, 2), (2, 2), (2, 1), (3, 3)}

Yes.

R = {(1, 3), (3, 2), (2, 1)}

No.

R = {(2, 4), (4, 3), (2, 3), (4, 1)}

No.


4 combining relations as sets

4. Combining Relations as Sets

  • Relations are sets, and therefore, we can apply the usual set operations to them.

  • If we have two relations R1 and R2, and both from a set A to set B, then we can combine them to obtain R1 R2, R1  R2, or R1 – R2.

  • The result us another relation from A to B.


5 composition of relations

5. Composition of Relations

  • Let R be a relation from a set A to a set B and S a relation from B to a set C.

  • The compositeof R and S is the relation consisting of ordered pairs (a, c), where aA, cC, when there is an element bB such that (a, b)R and (b, c)S.

  • We denote the composite of R and S by SR.

    • In other words, if relation R contains a pair (a, b) and relation S contains a pair (b, c), then SR contains a pair (a, c).


Example 1

Example 1

  • Let D and S be relations on A = {1, 2, 3, 4}

    D = {(a, b) | b = 5 - a} “b equals (5 – a)”

    S = {(a, b) | a < b} “a is smaller than b”

    D = {(1, 4), (2, 3), (3, 2), (4, 1)}

    S = {(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)}

    SD = { (2,4), (3,3), (3,4), (4,2), (4,3), (4,4) }

  • D maps an element a to the element (5 – a), and afterwards S maps (5 – a) to all elements larger than (5 – a), resulting in S°D = {(a,b) | b > 5 – a} or S°D = {(a,b) | a + b > 5}.


Example 2

Example 2

  • Let X and Y be relations on A = {1, 2, 3, …}

    X = {(a, b) | b = a + 1} “b equals a plus 1”

    Y = {(a, b) | b = 3a} “b equals 3 times a”

    X = {(1, 2), (2, 3), (3, 4), (4, 5), …}

    Y = {(1, 3), (2, 6), (3, 9), (4, 12), …}

    XY = { (1,4), (2,7), (3,10), (4, 13), ... }

  • Y maps an element a to 3a, and afterwards X maps 3a to 3a + 1

    XY = {(a,b) | b = 3a + 1}


6 n ary relations

6. N-ary Relations

  • To apply relations to databases, we generalize binary relations into n-ary relations.

  • Let A1, A2, …, An be sets. An n-ary relation on these sets is a subset of A1 x A2 x … x An.

  • The sets A1, A2, …, An are called the domains of the relation, and n is called its degree.


Example

Example

  • Let R = {(a, b, c) | (a = 2b) (b = 2c), with a, b, c  Z}

  • What is the degree of R?

    • The degree of R is 3, so its elements are triples

  • What are its domains?

    • Its domains are all the set of integers

  • Is (2, 4, 8) in R?No

  • Is (4, 2, 1) in R?Yes


7 databases and relations

7. Databases and Relations

  • The database representation based on relations is called therelational data model

    • A database consists of n-tuples called records, which are made up of fields

    • These fields are the entriesof the n-tuples

  • The relational data model represents a database as an n-ary relation

    • in other words, a database is a set of records


Example1

Example

  • Consider a database of students, whose records are represented as 4-tuples with the fields Student Name, ID Number, Department, and GPA:

    R = { (Ackermann, 00231455, CoE, 3.88),(Adams, 00888323, Physics, 3.45),(Chou, 00102147, CoE, 3.79),(Goodfriend, 00453876, Math, 3.45),(Rao, 00678543, Math, 3.90),(Stevens, 00786576, Psych, 2.99) }

  • Relations that represent databases are also called tables, since they are often displayed as tables.


Primary key

Primary Key

  • A domain of an n-ary relation is called a primary key if the n-tuples are uniquely determined by the values from this domain.

    • this means that no two records have the same primary key value

  • e.g., which of the fieldsStudent Name, ID Number, Department, and GPAare primary keys?

    • Student Name and ID Number are primary keys, because no two students have identical values in those fields

  • In a real student database, only ID Number would be a primary key


8 more information

8. More Information

  • Discrete Mathematics and its ApplicationsKenneth H. RosenMcGraw Hill, 2007, 7th edition

    • chapter 9, sections 9.1 – 9.2


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