chapter 2 ec601
TRANSCRIPT
-
8/10/2019 Chapter 2 EC601
1/43
-
8/10/2019 Chapter 2 EC601
2/43
-
8/10/2019 Chapter 2 EC601
3/43
Entity is an object of the real world which can
store data and can be defined with clarity. Example : something that exist physically such
as individual, place, article or something that
exists conceptually such as an event or concept
in a user environment. Each entity has attributes, or a particular
properties that describe the entities.
Example: student (entity) name Aminah has
attributes or properties of her own Student
Reg. No, name and grade
A particular value of an attribute, such as 93
for the grade is a value of the attribute.
-
8/10/2019 Chapter 2 EC601
4/43
Sometimes, the value of an attribute is
unknown or missing and sometimes a value isnot applicable.
In such cases, the attribute can have the
special value of null.
Example : until the teacher grades anassignment , the team grade is missing or null..
-
8/10/2019 Chapter 2 EC601
5/43
ENTITY EXAMPLE
invidual Customer, Worker, Supplier
venue / place District, State, Country
object Stationary, Furniture
event Purchase Order, Booking,Sale
concept Project, Account, Unit
-
8/10/2019 Chapter 2 EC601
6/43
The ER model consist of different types ofentities.
The existence of an entity may depends onthe existence of one or more other entities.
In other words : an entity is said to beexistence dependent.
Entities based on their characteristic areclassified as follows :
Strong entitiesWeak entities
Recursive entities
Composite entities
-
8/10/2019 Chapter 2 EC601
7/43
Weak Entity Type
An entity set that does not have sufficient
attribute to form a primary key is termed as
weak entity.
It is also a type that is existence-dependent onsome other entity type.
Strong Entity Type
Entity set that has a primary key is strong entity
set. Entity type that is notexistence-dependent on
some other entity type.
-
8/10/2019 Chapter 2 EC601
8/43
Recursive Entity
-A recursive entity is one in which a relation canexist between occurrences of the same entity
set. This occurs in a unary relationship.
Unary relationship is relationship between the
instances of a single entity type. (bothparticipants in the relationship are the same
entity)
Eg: employee to manager. Ones manager is, in most
cases also an employee with a manager of his own.
EMPLOYEE
Employee_ID Name Birthdate Manager_ID
-
8/10/2019 Chapter 2 EC601
9/43
Composite Entity-If a Many to Many relationship exist we must
create a bridge entity to convert it into 1 to
Many.
Bridge entity composed of the primary keys
of each of the entities to be connected.
The bridge entity is known as a composite
entity. A composite entity is represented by a
diamond shape with in a rectangle in an ER
Diagram.
-
8/10/2019 Chapter 2 EC601
10/43
Eg: in a school database, a TEACHER can
teach many different SUBJECTs and aSUBJECT can be taught by many differentTEACHERs.
Such situations are modelled using a
composite entity, which is usuallydecomposed to several one to manyrelationship later in the modelling process.
The composite entity must contain theprimary keys of the associated entities as its
foreign keys. Represent using a diamond in a rectangle.
-
8/10/2019 Chapter 2 EC601
11/43
SUBJECTTEACHER teachteach
LNameFName
Ext
Subj_code
-
8/10/2019 Chapter 2 EC601
12/43
-
8/10/2019 Chapter 2 EC601
13/43
-
8/10/2019 Chapter 2 EC601
14/43
Entity Set
An entity may be concrete such as a person,book etc or may be abstract such as account,loan etc.
The ER model refers to a specific table row as anentity instance or entity occurrence.
Collection of similar entities (Entity Set) oftencorresponds to a table.
Each entity set has a key.
All entities in an entity set have the same set ofattributes. Thus entity set is a set of entities ofthe same type that share the same properties orattributes.
An entity is represented bya rectangle containing the entity name, which isa noun usually written in capital letters.
-
8/10/2019 Chapter 2 EC601
15/43
A description or an entity characteristic isknown as an attribute.
Used to differentiate object in an entity.
The symbol used to represent attribute isoval and connected to entities by lines
-
8/10/2019 Chapter 2 EC601
16/43
-
8/10/2019 Chapter 2 EC601
17/43
Explain the attribute and a value set.
Explain E-R model.
Explain the following relationship types in E-
R model : 1:1 (one to one)
1:M (one to many)
M:N (many to many)
Draw E-R model based on a given situation.
-
8/10/2019 Chapter 2 EC601
18/43
Simple Attribute has only one component,exist independently and cannot be brokenup. Eg: name or gender
Composite Attribute comprises of manycomponents, each one existingindependently. Eg: address with sub attributes such as
o house_no,
o road_no,
o town
o postcode
-
8/10/2019 Chapter 2 EC601
19/43
Solitary Valued Attributeis an attribute thatconsists of only a single value. (matric_no, ic_no )
Multi-Valued Attribute is an attributeconsisting of many values. ( telephone no)
Derived Attributeis an attribute where its
values is derived from the value of relatedattributes or set of other attributes.( ageattribute derived from the date of birth )
-
8/10/2019 Chapter 2 EC601
20/43
[ composite attribute ]
[ multi-valued attribute]
[ derived attribute]
[ simple attribute]
STUDENT
matricno
age
address
dateofbirth
gender
studentname
postcode
garden
road
houseno
phoneno
specialty
-
8/10/2019 Chapter 2 EC601
21/43
You share many relationships with members
of your family.
For instance, you and your mother are
relatedYou have only one mother, but she
may have several children.
You and your siblings are related you may
have many brothers and sisters and, of
course, they'll have many brothers and
sisters as well. If you're married, both you and your spouse
have a spouseeach otherbut only one at a
time.
-
8/10/2019 Chapter 2 EC601
22/43
So in database, relationship is basically a set
of meaningful association between tables
A relationship exists between two database
tables when one table has a foreign key thatreferences the primary key of another table.
-
8/10/2019 Chapter 2 EC601
23/43
E-R Model is a conceptual and semantic
model of a database that captures meanings
rather than an actual implementation.
Use E-R model to get a high-level graphicalview of essential components of your
database and how they are related
So, the E-R Model is viewed as set of
Entities Relationships among the entities
-
8/10/2019 Chapter 2 EC601
24/43
Symbols used in E-R Diagram
Entityrectangle
Attribute oval
Relationship diamond
Link - line
-
8/10/2019 Chapter 2 EC601
25/43
TYPES INER MODEL
ONE TOONE
ONE TOMANY
MANY TOMANY
-
8/10/2019 Chapter 2 EC601
26/43
-
8/10/2019 Chapter 2 EC601
27/43
One to one 1 :1
This type of relationship takes place when a
single occurrence of an entity is related to just
one occurrence of a second entity.
For example, a ROOF covers one BUILDING; aBUILDING is covered by one ROOF.
In other words, both tables can have only one
record on either side of the relationship. Each
primary key value relates to only one (or no)record in the related table.
-
8/10/2019 Chapter 2 EC601
28/43
-
8/10/2019 Chapter 2 EC601
29/43
-
8/10/2019 Chapter 2 EC601
30/43
One-to-many 1:M
Relationships occur when each record in TableA
may have many linked records in TableB but
each record in TableB may have only onecorresponding record in TableA.
This relationship is similar to the one between
you and a parent. You have only one mother, but
your mother may have several children.
-
8/10/2019 Chapter 2 EC601
31/43
-
8/10/2019 Chapter 2 EC601
32/43
-
8/10/2019 Chapter 2 EC601
33/43
Many to manyM: N
Relationship where multiple rows from table A can
correspond to multiple rows in table B.
An example of such a relationship is a schoolwhere teachers teach students.
In most schools each teacher can teach multiple
students and each student can be taught by
multiple teachers.Other example; if you have several siblings, so do
your siblings (have many siblings).
-
8/10/2019 Chapter 2 EC601
34/43
-
8/10/2019 Chapter 2 EC601
35/43
EXAMPLE II
A customer is associated with several (possibly 0)
loans via borrower A loan is associated with several (possibly 0)
customers via borrower
-
8/10/2019 Chapter 2 EC601
36/43
-
8/10/2019 Chapter 2 EC601
37/43
-
8/10/2019 Chapter 2 EC601
38/43
The relationship type orderconnects
entities chemicaland supplier.
The relationship is many-to-many because
each chemical can be from several suppliers
and each supplier has a number of
chemicals.
-
8/10/2019 Chapter 2 EC601
39/43
An order has a purchase date, amount, and
total cost as well as the chemical and
supplier information.
Thus, orderhas attributes PurchaseDate,
amount, and TotalCostthat we cannot
appropriately associate
with chemicalor supplier.
-
8/10/2019 Chapter 2 EC601
40/43
-
8/10/2019 Chapter 2 EC601
41/43
Describe the relationship for each E-R model. Eg 1:
Situation : An employee has a single resume;
each resume belongs to only one person.
Description : The 1:1 relationship reflects the rules that
an employee can have only one resume and that
a resume can belong to only one employee.
-
8/10/2019 Chapter 2 EC601
42/43
Eg 2:
Description: A department can have many employees,
but that each individual employee can work
for only one department.
1 M
-
8/10/2019 Chapter 2 EC601
43/43
Eg 3:
Description: An employee can work on more than one project,
and a project can have more than one employee assigned.
M1