ghani dbms _entity relationship model
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8/7/2019 Ghani DBMS _Entity Relationship Model
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 1
Entity Relationship Model
Conceptual Design is an important phase in designing a successful database application
Without conceptual models, it is very difficult to communicate database designs to
(nontechnical) users - This lack of communication may result in users data requirementsbeing missed or incorrect requirements being captured
The Entity Relationship (ER) Model is one of the most widely used method for conceptualdesign.
The ER Model can be neatly mapped to a Relational SchemaModule 3 Contents:
The Entity-Relationship Model
Conceptual Design and how conceptual design relates to the database design process Entity-Relationship (ER) Conceptual Design Method Notation Guide Example Illustrations
Phases of Database Design:
First step of the design process is to identify the Universe of Discourse (UoD) The database to be built will not model everything in the world, but rather some mini-world
or Universe of Discourse.
The UoD is the relevant portion of the real world to be modeled by the database
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 2
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 3
Module 3 Contents:
The Entity-Relationship Model
Conceptual Design and how conceptual design relates to the database design process Entity-Relationship (ER) Conceptual Design Method Notation Guide Example Illustrations
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 4
ERModel Basics:
Entities Entities, Entity Sets and Entity Types
Attributes, Keys and Value Sets
RelationshipsRelationship Types and Sets
Relationship Degree
Roles andRecursive Relationships
Relationship Constraints
Attributes ofRelationship Types
Entities:
Entity Entity Sets Entity Type Attributes (Types, Keys and Value Sets)
Real-world object distinguishable from other objects
(e.g a student, car, job, subject, building ...)
An entity is described using a set of attributes The same entity may have different prominence in different UoDs
In the Company database, an employees car is of
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 5
lesser importance
In the Department of Transportations registration
database, cars may be the most important concept
In both cases, cars will be represented as entities;
but with different levels of detail
Entity Sets:
A collection of similar entities (e.g. all employees)
All entities in an entity set have the same set of attributes Each entity set has a key Each attribute has a domain Can map entity set to a relation easily
Entity Type:
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 6
Attributes:
Value Sets ofAttributes:
Value sets specify the set of values that may be assigned to a particular attribute of an entity Employee Age: Integers between 21 & 65
Vehicle Registration Number: String of 3
alphabets followed by 3 integers
Value sets map to relational domains Value sets are not displayed on the ER diagram
Null Valued Attributes:
A particular entity may not have an applicable value for an attribute Tertiary-Degree: Not applicable for a person
with no university education
Home-Phone: Not known if it exists
Height: Not known at present time
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 7
Type of Null Values Not Applicable
Unknown
Missing
Composite Vs. Simple Attributes:
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 8
ER Model Basics:
Entities Entities, Entity Sets and Entity Types
Attributes, Keys and Value Sets
Relationships Relationship Types and Sets
Relationship Degree
Roles and Recursive Relationships
Relationship Constraints
Attributes of Relationship Types
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 9
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 10
Relationship Constraints:
What are Relationship Constraints ?
Constraints on relationships are determined by the UoD, which these relationships aredescribing Constraints on the relationship type limit the possible combination of entities that may
participate in the corresponding relationship set
Kinds of Constraints:
What kind of constraints can be defined in the ER Model?
Cardinality Constraints Participation Constraints
Together called Structural Constraints
Constraints are represented by
specific notation in the ER diagram
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 11
Example Cardinality Constraints:
How many Employees can work in a Department?
One employee can work in only one department
How many Employees can be employed by a Department?
One department can employ many employees
How many managers can a department have?
One department can have only one manager
How many departments can an employee manage?
One employee can have manage only one department
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 12
Kinds of participating constraints:
TOTAL Participation (Existence Dependency) Constraint : Every employee must work for a department
PARTIAL Participation Constraint : Not every employee is a manager
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 13
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 14
WeakEntities:
Owner and weak entity must participate in a 1:N relationship type Weak entity must have total participation in the identifying relationship set (existence
dependency)
Weak entities normally have a Partial Key, which is a set of attributes that uniquely identifyweak entities related to the same owner entity
Not every existence dependency results in a weak entity !
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 15
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 17
Example Illustrations:
Project Management Course Administration Recruitment Company Database
Note how different (interpretations of) semantics change the ER Diagram
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Entity Relationship Model
Abdul Ghani Khan, M.Tech (ICT)
School of ICT, Guatam Buddha University Page 18
Company Schema (Relational):
EMPLOYEE[Ssn, Fname, Minit, Lname, Bdate, Address, Sex, Salary , SuperSsn ] DEPARTMENT[ Dnumber, Dname MGRSSN, MgrStart ] PROJECT[Pno, PName, Plocation, Dnum ] DEPENDENT[ ESSN,DepName, Sex, BirthDate, Relationship ] WORKS_ON[ ESSN, PNo, Hours ] DEPT_LOCS[ DNumber, Dlocation ]