Lecture Database management systems Chapter 4 Relational model

Chapter 4 Relational model. The main contents of this chapter include all of the following Relational model concept, relational data model, relational database, relation schema, relational algebra, relational query languages,...

Chapter 4

Relational Model

Relational Model Concept

 Relational database: a set of relations

 Each relation resembles a table of values

 Relation: made up of 2 parts

◦ Instance: a table, with rows and columns

◦ Schema: specifies name of relation, plus name

and type of each column

 The terms commonly used User Model Programmer

Column Attribute Field

Table Relation File

Relational Data Model

 The relational model uses a collection of

tables to represent both data and the relationships among those data

 Each relation resembles a table of values

 Tables are logical structures maintained

by the database manager

 Ex:

STUDENT (NO., NAME, DATE_OF_BIRTH, GENDER)

Relation Set of attributes

Relational Database

 Ex:

◦ A row is called tuple

◦ A column is called attribute

◦ The table is called relation.

Relational Database

 For each column of a table, there is a set

of possible values called its domain

 Domain is the set of valid values for an attribute

Relation Schema

 Relation Schema R, denoted by R(A1, A2,…, An), is made up of relation name R and a list of attributes A1, A2, …,An

 Each attribute Ai is the name of a role played by some domain D in the relation schema R

 D is called the domain of Ai and is denoted by dom(Ai)

 R is called the name of the relation

 The degree of a relation is the number of attributes n of its relation schema

Relational Data Model

 The relational model is a combination of three components:

◦ Structural Part: the database as a collection

of relations

◦ Integrity Part: maintained in the relational

model using primary and foreign keys

and relational calculus are the tools used to manipulate data in the database

Concept of Key

 An attribute or group of attributes, which

is used to identify a row in a relation

 Can be classified into 3 types:

◦ Super key

◦ Candidate key

◦ Primary key

Concept of Key

 Candidate key:

◦ a minimal super key

◦ a minimal set of attributes whose values uniquely identify tuples in the corresponding relation

◦ employee salary, etc

 Super keys can be: employee ID, employee name, employee age, employee experience, etc

 Candidate keys can be: employee ID, employee name, employee age

 Primary key: employee ID

Concept of Key

Relational Query Languages

 A major strength of the relational model: supports simple, powerful querying of data

 Queries can be written intuitively, and the DBMS is responsible for efficient evaluation

 The SQL Query Language is developed

by IBM in the 1970s

◦ If PLAYER’s name’s used as the primary key

 cannot insert any data in the relation PLAYER without entering the name of the player

non-Relational Integrity

 Domain Integrity Constraint

◦ are based upon the semantics of the real world enterprise that is being described in the database relations

◦ are used in the relational model to define the characteristics of the columns of a table

 Ex:

◦ The age of the person cannot have any letter from the alphabet The age should be a numerical value

Relational Integrity

 Referential Integrity

◦ a database must not contain any unmatched foreign key values

◦ not imply a foreign key cannot be null

◦ Either each foreign key value must match a primary key value in another relation or the foreign key value must be null

ER design to Relational

 Entity sets to tables

 EMPLOYEE (Emp_ID, Name, Age)

CREATE TABLE Employees

(Emp_ID CHAR(11), Name CHAR(20), Age INTEGER, PRIMARY KEY (Emp_ID)) EMPLOYEE

Emp_ID

Name

Age

Relational Algebra

 Query languages: Allow manipulation and

retrieval of data from a database

 Relational model supports simple, powerful QLs:

◦ Strong formal foundation based on logic

◦ Allows for much optimization

 Query Languages != programming languages!

◦ QLs not intended to be used for complex calculations

◦ QLs support easy, efficient access to large data sets

Relational Algebra

Relational algebra

* Selection

* Projection

Selection

 Work on a single relation R

 Define a relation that contains only those tuples of R that satisfy the specified condition

Selection

 Ex:

 Selection operation is associative

Selection _ example

 List all students who get GPA mark > 8

Student Roll No Name GPA

Projection

 Define a relation that contains a vertical subject of R, extracting the values of specified attributes and elimination duplicates

 Syntax: pa1,a2, ,an( R)

 pName, salary ( STAFF)

 Give the name and Date of birth of the all the staff

 pName, Date of birth ( STAFF)

 Define a relation consisting of the set of all tuples that are in both R and S

 Expression: R S

 Ex:

Intersection

R  S = { t | tR  tS }

 Define a relation consisting of the tuples that arein relation R but not in S

 Expression: R - S

 Ex:

Intersection

R - S = { t | tR  tS }

◦ Expression: Original relation: R(A, B, C)

◦ Attribute A will be change into X

ρS(R)

ρX,B,C(R)

◦ E mp_Dep4  σDepNo=4 (STAFF))

◦ R  pFName, LName ( Emp_Dep4 )

◦ ρFirstName, LastName(R)

Cartesian Product

 Define a relation that is the concatenation

of every tuples of relation R with every tuples of relation S

 The result of Cartesian product contains

all attributes from both relations R and S

 Expression: R x S

Cartesian Product

 Ex:

Cartesian Product

 Ex:

Cartesian Product

 Ex:

 3 types of join operation:

◦ Natural Join

◦ Equi Join

◦ Theta Join

Equi Join

 A special case of condition joins where the condition C contains only equality

 Expression:

Equi Join

◦ Enforce equality on all column attributes

◦ Eliminate one copy of common attribute

Natural Join

Theta Join

 A conditional join in which we impose condition other than equality condition

Outer Join

 In outer join, matched pairs are retained unmatched values in other tables are left null

 Full outer join

 Left outer join

 Right outer join

Outer Join

Outer Join

Outer Join

Outer Join