Relational Data Model

Sahar Mosleh California State University San Marcos Page 1

Relational Data Model


Data Model

• Structures and access techniques provided by a particular database management system (DBMS) are called its data model

• DBMS became popular from 1970 to 1980

• Each model had its advantages and its disadvantages

• In many ways the relational data model represented an attempt to simplify the earlier data models


File Management Systems• Before the existence of DBMS, the data were stored in

separate files.

• There was no link from one file to another

• If the structure of the data changed (ex: adding more fields), programs that were using the file had to change

• Problems became more severe when the number of the programs using the files increased over time


Employee Update Program

FD

Check-writing Program

FD

FD

EmployeeMaster

File

TimecardFile

Employee Report Program

FD

• This is an example of payroll application using a file management System. (FD is the file descriptor in the picture)

EmpId EmpName Address City ……………….


EmpId HoursWorked Wage ……………….



Hierarchical Database

• One of the most important application for the earliest database management system was production of manufacturing companies such as car companies

• Car companies wanted to produce 10000 units of one model, 5000 of another model and so on.

• They needed a database to collect all of this information and required tools to retrieve the information as fast as possible.

• Information had to be organized in a form of parent and child structure:

• a car is composed of a motor, body , etc. • Similarly each subpart were composed of other subparts and so

on. Ex: an engine has spark plugs, wires, etc..


Car1

Engine

Left Door

Handle

…...

……

Body Transmission

right Door

RoofHood

Window Lock

Car2

Engine

Left Door

Handle

…...

……

Body Transmission

right Door

RoofHood

Window Lock

Car3

Engine

Left Door

Handle

…...

……

Body Transmission

right Door

RoofHood

Window Lock

…..…..…..…..…..…..…..…..….….


• Hierarchical model uses upside down trees.

• A tree represents parent/child relationships• For example, a car consists of body, engine, transmission, etc.

• The root of the tree and all of its associated children would make one record (specifying a particular item: i.e a car)

• Pointers were used to link a parent to its children or a child to another child

• In general, to access the data, a program would • Find a particular car • Move down to the first child• Move up to its parent• Move sideways to the next child

• Retrieving the data in a hierarchical database required navigating through the records, moving up, down, and sideways one record at the time

• The most popular hierarchical database was Information Management System (IMS) introduced in 1968


Advantages of Hierarchical database:• Simple Structure:

• Organization of IMS database was easy to understand

• Parent/Child Organization:• IMS was a good model for representing parent/child

relationship (A is part of B) or (A is owned by B)

• Performance:• IMS stored parent/child relationships as physical

pointers from one data record to another• This would speed up the movement from one record to

another

• IMS is still widely used. It is still considered as a good DBMS in places with lots of transactions processing (ex: banks)


Network Databases

• Hierarchical database could not answer the demand of some business oriented environment.

• For example, in an order processing company, a single order might participate in more than one parent/child relationship.

• For instance, a particular order should be linked to • The customer who placed it• The sales person who took it• The product ordered

• This could not be done by IMS

• To deal with these situations, network data model was developed: children could have more than one parent


CarCar

#112963

CarCarBill

Adams

CarCarAcme

Mfg

CarCarSize 4

Widget

...

... ......

Orders

Customers Salespeople Products

• Example of parent/child relationship in network database models


• In 1971, the conference on the systems languages published an official standard for network databases which became known as CODASYL model

• A programmers would access the network database as follows:

• Find a specific parent record by key (ex: customer number)

• Move down to the first child in a particular set (the first order placed by this customer

• Move sideways from one child to the next in the set (the next order placed by this customer)

• Move up from a child to its parent in another set ( the salesperson who took the order)


Advantages of Network database

• Flexibility:• Multiple parent/child relationships allowed a network database

to represent data that did not have a simple hierarchical structure

• Standardization:• The CODASYL standard boosted the popularity of the network

model, and minicomputer vendors such as Digital Equipment Corporation and Data General implemented network Databases

• Performance:• Despite their greater complexity, network databases boasted

performance approaching that of hierarchical databases.


Disadvantages of Hierarchical and Network models

• They have rigid structure:

• The structure of the records had to be known in advance.

• Changing the database structure required rebuilding the entire database

• Querying the database was not always easy. Retrieving simple information form the database could cause programmer to write lots of code

• Some of this code was quite complicated


Relational Data Model• Disadvantages of hierarchical and network models led to

creation of Relational Data Model by Dr. Codd in 1970

• In this course, we teach Relational data model and use Structured Query Language (SQL) used to manipulate the data in the database.

Definition:• A relational database is a database where all data visible to

users is organized strictly as tables of data values and where all database operations work on these tables

• In this model information is stored in a database as simple row/column tables of data

• Next slide shows an example of tables in a relational database


Salesrep Name Age RepOffice Title HireDate Manager Quota Sales105 Bill Adams 37 13 Sales Rep 12-FEB-88 104 350000 367911109 Mary Jones 31 11 Sales Rep 12-OCT-89 106 300000 392725102 Sue Smith 48 21 Sales Rep 10-DEC-86 108 350000 474050106 Sam Clark 52 11 VP Sales 14-JUN-88 275000 299912104 Bob Smith 33 12 Sales Mgr 19-MAY-87 106 200000 142594101 Dan Roberts 45 12 Sales Rep 20-OCT-86 104 300000 305673110 Tom Synder 41 Sales Rep 13-JAN-90 101 75985108 Larry Fitch 62 21 Sales Mgr 12-OCT-89 106 350000 361865103 Paul Cruz 29 12 Sales Rep 01-MAR-87 104 275000 286775

1 Nacy Angelli 49 22 Sales Rep 14-NOV-88 108 300000 186042 ….….

SalesReps

OrderNum OrderDate Cust Rep MFR Product QTY Amount 112961 17-DEC-89 2117 106 REI 2A44L 7 31500 113012 11-JAN-90 2111 105 ACI 41003 35 3745 112989 03-JAN-90 2101 106 FEA 114 6 1458 113051 10-FEB-90 2118 108 QSA K47 4 1420 112968 12-OCT-89 2102 101 ACI 41004 34 3978 113036 30-JAN-90 2107 110 ACI 4100Z 9 22500 113045 02-FEB-90 2112 108 REI 2A44R 10 45000 112963 17-DEC-89 2103 105 ACI 41004 28 3276 113013 14-JAN-90 2118 108 BIC 41003 1 652 113058 23-FEB-90 2108 109 FEA 112 10 1480 112997 08-JAN-90 2124 107 BIC 41003 1 652 112983 27-DEC-89 2103 105 ACI 41004 6 702 113024 20-JAN-90 2114 108 QSA XK47 20 7100 113062 24-FEB-90 2124 107 FEA 114 10 2430 112979 12-OCT-89 2114 102 ACI 4100Z 6 15000 ……

Orders

MfrId ProductId Description Price QtyOnHandREI 2A45C RATCHET LINK 79 210ACI 4100Y WIDGET REMOVER 2750 25QSA XK47 REDUCER 355 38BIC 41672 PLATE 180 0IMM 779C 900-LB BRACE 1875 9ACI 41003 SIZE 3 WIDGET 107 207ACI 41004 SIZE 4 WIDGET 117 139BIC 41003 HANDLE 652 3IMM 887P BRACE PIN 250 24QSA XK48 REDUCER 134 203REI 2A44L LEFT HINGE 4500 12….

Products

CustNum Company CustRep Credit_limit 2111 JCP Inc. 103 50000 2102 First Corp. 101 65000 2103 Acme Mfg. 105 50000 2123 Carter and Sons 102 40000 2107 Ace International 110 35000 2115 Smithson Corp. 101 20000 2101 Jones Mfg. 106 65000 2112 Zetacorp 108 50000 2121 QMA Assoc. 103 45000 2114 Orion Corp. 102 20000 ……

Customers

OfficesOffice City Region Mgr Target Sales 22 Denver Western 108 300000 186042 11 New York Eastern 106 575000 692637 12 Chicago Eastern 104 800000 735042 13 Atlanta Eastern 105 350000 367911 21 Los Angeles Western 108 725000 835915


Table

Definition:• A table is a rectangular object with rows and columns

• For example in the office table:• Each row of the office table represents a single physical entity• Each column of the offices table represents one item of data

that is stored in the database for each office:• Ex: City column represents the location of the office

• An alternative term for column is attribute

• Each row of the table contains exactly one data value in each column


• In each column of a table, all of the data values in that column have the same type. For example:

• City column values are words• Sales values are money type• Mgr values are integer

• Each column in a table has a column name which is written as a heading at the top of the column

• Column names must be unique in a table

• The columns of a table have a left-right order. That is defined when the table is first created.

• The order of the column has no effect when any action is done against the table


• Each table must have at least one column

• Almost all commercial DBMS products impose maximum of 255 columns per table

• A table can have zero or more rows

• A table with zero rows is called an empty table

• Order of the rows is not important in a table.

• Most relational DBMSs either do not impose any limit on the number of rows or their limit is a very large number

• A common limit is approximately 2 billion rows


Primary Key

Definition:• A primary key is a column or combination of two or more

columns that uniquely identifies each row of a table.

• Since the order of rows in a table is irrelevant, rows cannot be identified based on their positions in a table

• Ex: row 1, row 2, row 20

• In a well-designed relational database each table has a primary key.

• If the primary key contains two or more columns, it is called a composite primary key


Example of primary key

• Consider the Offices table

• “Office” column (attribute) can be a good choice for the primary key because each office has a different office id

• However, city is not a good choice because more than one office may be located in the same city.

Office City Region Mgr Target Sales 22 Denver Western 108 300000 186042 11 New York Eastern 106 575000 692637 12 Chicago Eastern 104 800000 735042 13 Atlanta Eastern 105 350000 367911 21 Los Angeles Western 108 725000 835915


• Consider the Products table

MfrId ProductId Description Price QtyOnHandREI 2A45C RATCHET LINK 79 210ACI 4100Y WIDGET REMOVER 27.50 25QSA XK47 REDUCER 355 38BIC 41672 PLATE 180 0IMM 779C 900-LB BRACE 1875 9ACI 41003 SIZE 3 WIDGET 107 207ACI 41004 SIZE 4 WIDGET 117 139BIC 41003 HANDLE 652 3IMM 887P BRACE PIN 250 24QSA XK48 REDUCER 134 203REI 2A44L LEFT HINGE 4500 12….

• What is a good primary key for this table?


• In his case, MrfId by itself, is not a good choice to be a primary key because more than one manufacturer may produce more than one product

• Further, ProductId by itself is not a good choice either because the same product can be produced by more than one manufacturer.

• However, combination of both is unique in every row.

• This is an example of composite primary key.

• A table with a primary key is called a relation. A relation is a table in which no duplicate rows can exist.


Relationship• In contrast with the older data models (hierarchical and

network models) relational data models do not include explicit parent/child pointer connection

• Then how does the parent/child model is represented in the relational data model?

• Consider the following two tables

Salesrep Name Age RepOffice Title HireDate Manager Quota Sales105 Bill Adams 37 13 Sales Rep 12-FEB-88 104 350000 367911109 Mary Jones 31 11 Sales Rep 12-OCT-89 106 300000 392725102 Sue Smith 48 21 Sales Rep 10-DEC-86 108 350000 474050106 Sam Clark 52 11 VP Sales 14-JUN-88 275000 299912….….

SalesReps

OfficesOffice City Regin Mgr Target Sales 22 Denver Western 108 300000 186042 11 New York Eastern 106 575000 692637 12 Chicago Eastern 104 800000 735042 13 Atlanta Eastern 105 350000 367911 21 Los Angeles Western 108 725000 835915


• The parent is the offices table

• The child is the salesreps table because the salesreps works in an office

• Relationships are created by having the same data in two or more tables


SalesReps



• Note that the RepOffice column salesreps table contains the office number of the sales office where each sales person works

• The values of the RepOffice column is the set of office numbers found in the office column of the offices table

• We will see how this restriction is imposed when we discuss about creating tables later in the course

• For example, it is possible to find the sales office where “Mary Jones” is working by finding the value of Mary Jones RepOffice (11) and finding the corresponding row offices table

• So, the parent/child relationship between two tables A and B is not represented by explicit pointers but by common data values stored in the two tables

• Programmers must specify this relationship when they create the tables


Foreign KeyDefinition:

• Foreign key is a column (or combination or two or more columns) whose value matches the primary key of another table

• Together, primary key and the foreign key make the parent/child relationship in relational data models


SalesReps


Primary key

Foreign key


Example of Foreign Key

Salesrep Name 105 Bill Adams109 Mary Jones102 Sue Smith106 Sam Clark….

SalesReps

OrderNum OrderDate Cust Rep MFR Product QTY Amount 11296 17-DEC-89 2117 106 REI 2A44L 7 31500 113012 11-JAN-90 2111 105 ACI 41003 35 3745 112989 03-JAN-90 2101 106 FEA 114 6 1458 113051 10-FEB-90 2118 108 QSA K47 4 1420 ….

Orders

MfrId ProductIdREI 2A45C …..ACI 4100Y …..QSA XK47 ……….…..

Products CustNum 2111 ….. 2102 …. 2103 …. ………. …….

Customers

Date post:	16-Mar-2016
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Relational Data Model

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