Data Modeling Basic

Copyright © 2004 AdventNet

Data Modeling

Basic Course


Some Definitions

Database: Storage mechanism for the information that we require

Relational database: A database consisting of more than one table and which are related in some way.

Data Model: Collection of description of tables together with the operations or functions that manipulate them in order to intelligently organize data.


Remember

Tables are also relations Tables are constructed and associated

to each other through shared fields Fields are also “columns” or

“attributes” Records are also “rows” or “tuples” Tables are related through common

fields designated as primary and foreign keys


Example

Create a table with following columns Employee name Date of Birth Address

Create table Employee(name varchar(20) not null, bdate date, address varchar(30));

Populate the table


SQL Queries

Select name from Employee where address like’%Velachery%’;

Insert into Employee values (‘vidhya’,1972-01-29’,’5A,Porur Street,East Tambaram,Chennai’);

Delete from Employee where name=‘vidhya’;

Update Employee set bdate=‘1999-02-9’ where name=‘vidhya’;


SQL Operations

Three basic update operations on relations are Insert- is used to add records Delete- removes record Update – can change an existing

record


Data Integrity Constraints

A limitation that you place on the data that users can enter into a row or column or groups of columns or table.

Constraints include: PK, UNIQUE, FK/REFERENCES, CHECK & DEFAULT Domain Constraints Entity Constraints Referential User Defined


Domain Integrity

Domain Constraints apply at column/ attribute level.

Ensures that set of data values fall within a specified range(domain) in order to be valid.

Examples: correct data type; values that fall within the range supported by the system; null status; permitted size values


Entity Integrity

Entity Integrity Constraint applies at the row level

Ensures that each row in a table is uniquely identified. For a car the license number is a key

as no two cars will have the same license number


Referential Integrity

Referential Integrity applies at table level

Referential integrity is typically enforced with a Primary Key (PK) and Foreign Key (FK) combination

Example- When an order is entered, the entered customer id must exist in customers.


Example

Let us add some more attributes to the Employee table- Date of Joining Team Name Manager Name Email address Phone number

Populate Data


Exercise

What are the constraints we would place for each attribute of the Employee table— Data type,size Nullable Unique PK/FK

Do this exercise again at the end of the course— Notice the changes..


Primary Key

The Primary Key is an Attribute or set of Attributes that uniquely identifies a specific instance of an Entity.

Primary Key field cannot contain a NULL value

Each Table should have a primary key A primary key that is made up of more

than one attribute is known as Composite Key


Primary key - Example

What is the primary key in our Employee table??

What if another employee with the same name joins the company?

We introduce <EmployeeID> as the primary key.


Foreign Key

A key used in one table to represent the value of a primary key in a related table.

While primary keys must contain unique values, foreign keys may have duplicates


Surrogate Key

A surrogate key is a key that has no meaning other then uniquely identifying a row in the table -it contains no meaningful business information.

Use Surrogate Keys to all “important” or core entities.

EmployeeID is a surrogate key.


NULL Avoidance

Databases allow NULLs, but they cause numerous problems, such as complicating the code – conditions like if (someAttribute != NULL)

AdventNet Rule: No NULLs allowed Use default values such as “No

Comment Entered” or “No Description Available” or “No Subject Provided”


Example

If an Employee does not have a phone number, what would we enter in the Employee table??

A “null” could mean that the phone number is not known to us or that the employee does not possess a phone.


Data Modelling – Why?

So that you can query and find what you want easily

To prevent bugs arising from inconsistent data

Well organized data greatly reduces and simplifies code needed for an application, and therefore improves quality

Database, XML are data storage mechanisms that should be backed up with well defined structure ie; data model


Elements of a Data Model

Entity - An Entity is a thing or object of importance about which data must be collected

Attribute- An attribute describes information about an entity that must be captured

Relationships- Represents an association between two or more entities


Entity

We have captured information (Name, Date of Birth, Address,Manager Name,Team,Email Address, Phone number) about each Employee and we cannot describe an employee without these items.

Hence, Employee is an Entity about which we want to collect data


Attribute

Name, Date of Birth, Address, Email Address, Team name, Manager, Phone number describe information about the employee

Hence, these are Attributes of the Employee table


Exercise

Make a list of the possible Entities, Attributes for the music CD’s that you have at home CD Name Movie Name Songs

Continue……


Entity Attribute Relationship

Name

Address

Owns/leasesMakeModelColour


Relationship

A relationship relates two or more distinct entities with a specific meaning.

Relationships are created between tables using the primary key field and a foreign key field.


Example

What is the relationship here- Employee ,PC Department, Employee Project, Employee


Example

An employee is assigned a PC, each PC is owned by an employee

An employee works in a department , a department has many employees

A project can have multiple employees working on it and each employee can work on multiple projects at the same time


Exercise

What is the relationship for the following- Student,class Customer,Order Sales representative, customers Movie,song


Solution

A student belongs to a class, a class has many students

A customer places an order or many orders, an order belongs to a customer

A sales representative meets many customers, a customer meets many sales representatives.

A movie contains many songs, a song belongs to a movie


ER Diagram

The model visually represents the Entity-Relationship concepts by the Entity-Relationship diagram.

The basic constructs of the ER model are entities, relationships, and attributes.


E1 E2

A1 A1 A2 A2

R

E-Entity

A-Attribute

R -Relationship

ER DiagramWe can express the overall structure of data

graphically with an ER Diagram


ER Diagram Example

One Room can hold many classes, but each class is taught in only one room

class room

No. building

seats

No.


Issues – In the Employee table

What if one employee belongs to multiple teams – such as during a transition to a new team?

What if an employee does not have a manager?


Example - Solution

Employee Table: EmployeeID, Name, Date Of Birth,

Date Of Joining, Email Address, Phone Number

Employee Team: EmployeeID, TeamID

Employee Manager: EmployeeID,ManagerID


Points to Remember

Uniquely identify records Eliminate Recurring Fields Eliminate Data Entry Anomalies Eliminate NULL entries


Example-Student-Activity

We’ll design a database to keep track of students’ sports activities. We’ll track each activity a student takes and the cost to do that activity.


Example- Student-Activity

Student name Activity Cost

John Smith Tennis $36

Jane Bloggs Badminton $40

John Smith Swimming $30

John Smith Tennis $35

Mark Antony Golf $60

Mark Antony Swimming $30


Student-Activity-Think

Redundant data entry If the tennis fees goes up to $39?

Redundant information If 50 students take swimming, we have to type in

both the activity and its cost each time Inconsistent data

Notice that there are conflicting prices for tennis Insertion anomalies

What if our school introduces a new activity, such as sailing, at $50.

Deletion anomalies If Mark Antony transfers to another school?


Solution to Student-Activity

Student(<StudentID>,FirstName,LastName)

Participant(<Student ID,Activity>) Activities(<Activity>,Cost)


Example-

Model the following- An academic department of a college

has the attributes Department name, Dept phone no., List of Faculty.

We want to store the following - City, street,house number, house colour, city population


Solution

1) Relation is 1:N <DepartmentID>, Dept Name, Dept

Phone no. <FacultyID>,DepartmentID

2) <CityID>, City Name, City population <AddressID>,house no., house colour,

streetID, CityID <streetID>,street name


See links

Bookstore

Airline


Quiz

A primary key is An attribute that can be used to identify a

single row in a table A key used in one table to relate to key in

another table An index for a table

A well defined data model allows us to Have a record of data Store data in a sequence Reduce amount of code needed for an

application


Quiz

Grades of a student should be A or B or C. You enter Z. What happens? The keyboard Z key gets locked An error message – constraint violated

appears on the screen Nothing

The three kinds of anomalies are insertion,selection,deletion insertion,update,deletion update, selection,deletion


How do I find

•Division 1 has Pat, Chris, and Jamie

•Division 1 operates branches 101 and 102

•Who works at branch 101?


Exercise

Model the following An Employee has the following details-

Name,Date of Birth, department name, skill set

The following details about a student is required- Student name, subject, marks

Restaurant order information needed-Customer Name, Address, Order Date, Item Name, Item Quantity

Online store customer information- First name, Last Name, address, city, state,Pin code


Guidelines

Every table should have a primary key NULL values not allowed Data should be present one and only

once Multiple values for an attribute not

allowed


Questions???

Data Modeling Basic

Documents

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