Week03 - The Relational Data Model

8/8/2019 Week03 - The Relational Data Model

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Database I

Background

The Relational Data Model


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History of Relational Data

Model

Proposed by E.F. Codd, in his research paper Arelational model of data for large shared data banksin 1970

Objectives of RDM: High degree of data independence Application program must not be affected by modifications

to the internal data representation

Provide environment for dealing with data consistency and

redundancy problems The first prototype of Relational Database

Management System (RDBMS) System R wasdeveloped by IBMs San Joes Research Laboratory

in late 1970s


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History of Relational Data

Model (Cont.)

System R project led to two major

developments: The development of SQL (pronounced as See-

Quel)

Production of various commercial RDBMS

products during 1980, i.e. DB2 and SQL/DS from

IBM and ORACLE from ORACLE Corporation


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Basic Terminology

Relation A relation is a table with columns and rows In relational model a database is perceived by the user as

tables

However this perception applies only to the logicalstructure i.e. external and conceptual levels

Attribute A named column of a relation Attributes can appear in any order and the relation will still

be the same relation Tuple Tuple is a row of a relation It is also called the extension or state of a relations which

changes over the time Tuples are also called records


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Basic Terminology (Cont.)

Domain A set of allowable values for one or more attributes

Structure of relation, together with the specification

of the domains is called intension of a relation

Degree The number of attributes a relation contains

A relation with only one attribute would have degreeone can be called a unary relation


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A relation with two attribute is called binary and

one with three attributes are called ternary, and

after that the term n-ary is used

Cardinality The number of tuples a relation contains

The cardinality is the property of extension of the

relation


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Examples of Attribute Domains


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Alternative Terminology

Formal Terms Alternative1 Alternative2

Relation Table File

Tuple Row Record

Attribute Column Field


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Mathematical Definition of

Relation

To understand the meaning of the term relation,

let us review some concepts from mathematics

Consider two sets, D1 & D2, where

D1 = {2, 4}

D2 = {1, 3, 5}

Cartesian product, D1 x D2, is set of all orderedpairs, where first element is member ofD1 and

second element is member ofD2D1 x D2 = {(2, 1), (2, 3), (2, 5), (4, 1), (4, 3), (4, 5)}


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Relation (Cont.)

Any subset of Cartesian product is a relation; e.g.

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

We can specify which pairs are in relation using

some condition for selection; e.g. Second element is 1:

R= {(x, y) |xD1, yD2, and y= 1}

First element is always twice the second:S= {(x, y) |x D1, yD2, andx= 2y}


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Relation (Cont.)

Consider three sets D1, D2, D3 with Cartesian

Product D1 x D2 x D3; e.g.

D1 = {1, 3} D2 = {2, 4} D3 = {5, 6}D1 x D2 x D3 = {(1,2,5), (1,2,6), (1,4,5),

(1,4,6), (3,2,5), (3,2,6), (3,4,5), (3,4,6)}

Again any subset of these ordered triples is arelation


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Relation (Cont.)

So Cartesian product ofn sets (D1, D2, . . ., Dn) is:

D1 x D2 x . . . x Dn = {(d1, d2, . . . , dn) | d1D1, d2D2, . . . , dnDn}

Usually written as:nX D

i

i = 1

Any set ofn-tuples from this Cartesian product is a

relation on the n sets


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Database Relations

Now applying the previous concepts to

database, we have:

Relation Schema: Relation is defined by a set of attribute anddomain name pairs

In our example, the branch relation on next slide

has four attributes with corresponding domains(tuples)

So the branch relation can be represented as:


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Database Relations (Cont.)

{(BNo:B005, Street:22 Deer Rd, City:London,

Postcode:SW1 4EH)}

This explains why

table rows in

relational model

are called tuples

Relational schema Set of relation schemas, each with a distinct name

Attribute Domain


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Properties of Relation

Relation name is distinct from all other relation

names in relational schema

Each cell of relation contains exactly one single

value Each attribute has a distinct name

Values of an attribute are all from the same domain

Each tuple is distinct; there are no duplicate tuples Order of attributes has no significance

Order of tuples has no significance, theoretically


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Relational Keys

Super Key An attribute or set of attributes that uniquely

identifies a tuple in the relation

Candidate Key A super key (K) such that no proper subset is a

super key within the relation

In each tuple of R, values of K uniquely identifythat tuple (uniqueness)

No proper subset of K has the uniqueness

property (irreducibility)


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Relational Keys (Cont.)

Composite Key A key which consists of more than one attribute

Primary Key Candidate key selected to identify tuples uniquely within

relation

Alternate Keys Candidate keys that are not selected to be primary key

Foreign Key An attribute, or set of attributes, within one relation that

matches candidate key of some (possibly same) relation


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Relational Integrity

Relational Integrity or Integrity Constraints are

set of rules, which insure that the data is

accurate

Null represents value for an attribute that iscurrently unknown or not applicable for tuple

Null is not the same as zero or spaces, which

are values Domain Constraints Restriction on the set of values allowed for the

attribute of relation


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Relational Integrity (Cont.)

Entity Integrity Applies to the primary keys of base relations

Base relation is corresponding to an entity in the

conceptual schema, whose tuples are storedphysically in database

According to entity integrity in a base relation, no

attribute of a primary key can be null

Referential Integrity The foreign key value must match a candidate key

value of some tuple in its home relation or foreign key

value must be null


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Relational Integrity (Cont.)

Enterprise Constraints Rules specified by the users or DBAs of a

database


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Views

A view is a Virtual Relation or in other wordsa relation that does not exist

So the external level of the three level

architecture can consists of both the baserelations and views

Contents of a view are defined as a query onone or more base relations

Views are dynamic, meaning that changesmade to base relations that affect viewattributes are immediately reflected in the

view


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Purpose of View

Provides powerful and flexible securitymechanism by hiding parts of database fromcertain users

Permits users to access data in a customizedway, so that same data can be seen bydifferent users in different ways, at same time

Can simplify complex operations on baserelations

All updates to a base relation should beimmediately reflected in all views that

reference that base relation and vice versa

Week03 - The Relational Data Model

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