Database Systems-Lec2

8/14/2019 Database Systems-Lec2

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Chapter 2The Relational DatabaseModel


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Entities and Attributes An entity is a person, place, event, or thing

for which we intend to collect data.

University -- Students, Faculty Members,Courses

Airlines -- Pilots, Aircraft, Routes, Suppliers

Each entity has certain characteristics knownas attributes.

Student -- Student Number, Name, GPA,Date of Enrollment, Data of Birth, HomeAddress, Phone Number, Major

Aircraft -- Aircraft Number, Date of LastMaintenance, Total Hours Flown, Hours

Flown since Last Maintenance

Entities & Attributes


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Entities and Attributes

A grouping of related entities becomes anentity set.

The STUDENT entity set contains allstudent entities.

The FACULTYentity set contains all

faculty entities.The AIRCRAFT entity set contains all

aircraft entities.



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Tables and Their Characteristics

A table contains a group of relatedentities -- i.e. an entity set.

The terms entity set and table are oftenused interchangeably.

A table is also called a relation.



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Fiel

d

Record

File

Common DataElementsAttribute

Entity Occurrenc

Entity Class Conceptual data modeling notationFile processing Notation

Student File

Id No. Name Faculty Major 1100 Ali FIT SE

1200 Bobby FIT MIS

1300 Clement FCM MM

1400 David FCM MM

1500 Evelyn FOE CE


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Fiel

d

Record

File

Attribute

Entity Occurrenc

Entity Class

Column

Row

Table

Conceptual data modeling notation

File processing Notation

Relational Table Notation

Student File

Id No. Name Faculty Major 1100 Ali FIT SE

1200 Bobby FIT MIS

1300 Clement FCM MM

1400 David FCM MM

1500 Evelyn FOE CE

Common DataElements


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Student Table


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Keys Controlled redundancy (shared common

attributes) makes the relational databasework.

The primary key of one table appears

again as the link (foreign key) in anothertable.

If the foreign key contains either matching

values or nulls, the table(s) that make useof such a foreign key are said to exhibit


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A Simple Relational Database


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The Relational Schema


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A key helps define entity relationships.

The keys role is based on a concept known asdetermination, which is used in the definition offunctional dependence.

The attribute B is functionally dependent on Aif A determines B.

An attribute that is part of a key is known as akey attribute.

A multi-attribute key is known as a compositekey.

If the attribute (B) is functionally dependent ona composite key (A) but not on any subset of

that composite key, the attribute (B) is fullyfunctionall de endent on (A).

Keys


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Relational Database Keys

Superkey

Uniquely identifies each entity STU_NUM

STU_NUM, STU_LNAME

STU_NUM, STU_LNAME, STU_INIT

Candidate key Minimal superkey STU_NUM

STU_LNAME, STU_FNAME, STU_INIT,

STU_PHONE


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Relational Database Keys

Secondary key

Used only for data retrieval Foreign key

Values must match primary key inanother table


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Integrity Rules

Entity integrity

Ensures all entities are unique Each entity has unique key

Referential integrity

Foreign key must have null value ormatch primary key values

Makes it impossible to delete rowwhose primary key has mandatory

matching foreign key values in


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An Illustration of IntegrityRules


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Relational Database Operators

The degree of relational

completeness can be defined by theextent to which relational algebra issupported.

Relational algebra defines thetheoretical way of manipulatingtable contents using the eightrelational functions: SELECT,

PROJECT,JOIN, INTERSECT, UNION,DIFFERENCE PRODUCT and DIVIDE.


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UNION

UNION combines all rows from two tables.The two tables must be union compatible(same attribute characteristics)


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INTERSECT

INTERSECT produces a listing that containsonly the rows that appear in both tables.The two tables must be union compatible.


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DIFFERENCE

DIFFERENCE yields all rows in one tablethat are not found in the other table; i.e.,it subtracts one table from the other. Thetables must be union compatible.


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SELECT

SELECT yields values for all attributesfound in a table. It yields a horizontalsubset of a table.


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PROJECT

PROJECT produces a list of all values forselected attributes. It yields a verticalsubset of a table.


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JOIN

JOIN allows us to combine information fromtwo or more tables.JOIN is the real powerbehind the relational database, allowingthe use of independent tables linked by

common attributes.


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Natural JOIN links tables by selecting

only the rows with common values intheir common attribute(s). It is theresult of a three-stage process:

A PRODUCT of the tables is created.(Figure 3.12)

A SELECT is performed on the outputof the first step to yield only the rows

for which the common attributevalues match. (Figure 3.13)

A PROJECT is performed to yield asingle copy of each attribute, thereby

eliminating the duplicate column.

Natural JOIN


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Natural Join, Step 1: PRODUCT


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Step 2: SELECT


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Step 3: PROJECT


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EquiJOIN links tables based on an equalitycondition that compares specifiedcolumns of each table. The outcome ofthe EquiJOIN does not eliminate duplicatecolumns and the condition or criteria to

join the tables must be explicitly defined.

Theta JOIN is an equiJOIN that comparesspecified columns of each table using acomparison operator other than theequality comparison operator.

In an Outer JOIN, the unmatched pairswould be retained and the values for the

unmatched other tables would be left

EquiJOIN & Theta JOIN


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OUTER JOIN In outer join for tables CUSTOMER and

AGENT, two scenarios are possible.

- Left outer join

Yields all rows in CUSTOMER table,including those

that do not have a matching values inthe AGENT table


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OUTER JOIN- Right outer join

Yields all rows in AGENT table, includingthose

that do not have a matching values inthe CUSTOMER table


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DIVIDE

DIVIDE requires the use of one single-column table and one two-column table.


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The Data Dictionary and theSystem Catalog

Data dictionary contains metadata toprovide detailed accounting of all tableswithin the database.

System catalog is a very detailed systemdata dictionary that describes all objectswithin the database.

System catalog is a system-createddatabase whose tables store thedatabase characteristics and contents.

System catalog tables can be queried justlike any other tables.


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A Sample Data Dictionary


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Relationships within theRelational Database

E-R Diagram (ERD)

Rectangles are used to represent

entities. Entity names are nouns and capitalized.

Diamonds are used to represent therelationship(s) between the entities.

The number 1 is used to represent the1 side of the relationship.

The letter M is used to represent the

many sides of the relationship.

Relationships within the


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1:M relationship

- Relational modeling ideal

- Should be the norm in any relationaldatabase design

M:N relationship

- Must be avoided because they lead to data

redundancies

1:1 relationship

- Should be rare in any relational database

design

Relationships within theRelational Database


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The 1:M Relationship Between

Painter and Painting


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The M:N Relationship Between

Student and Class


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Sample Student Enrollment Data


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A Many-to-Many Relationship

Between Student and Class


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The Expanded

EntityRelationshipModel


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The Relational Schema for the

Entity Relationship Diagram


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Data Redundancy Revisited

Data redundancy leads to data

anomalies, such anomalies candestroy database effectiveness.

Foreign keys control redundancies

by using common attributes sharedby tables

Sometimes, data redundancy is

necessary


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The redundancy is crucial to the systems success.Copying the product price from the PRODUCT table tothe LINE table means that it is possible to maintain the

historical accuracy of the transactions.

The Relational Schema for

the Invoicing System


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References

ROB, P. AND CORONEL, C., 2004, Database

Systems. 6th Ed., Thomson Course Technology

Documents

Database Systems-Lec2