Database design, E-R Model

1. INT306
Database Management Systems
DATABASE DESIGN & THE E-R MODEL
Text book
Database System Concepts
A. Silberschatz, H. F. Korth, S. Sudarshan
Course Instructor
Mr. Sumit Mittu
Assistant Professor and Placement Coordinator, CSE/IT
Lovely Professional University, Punjab (India)
sumit.12735@lpu.co.in sumit.mittu@gmail.com
http://tinyurl.com/askSumit
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2. IN THIS CHAPTER
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data modelling
overview of database design process
entity-relationship model
constraints
removing redundant attributes in entity sets
entity-relationship diagram
reduction to relational schema
entity-relationship design issues
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3. DATA MODELS
• Data Model defined…
• Collection of conceptual tools for describing:
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Data
Data relationships
Data semantics, and
Consistency constraints
• Types
• Object Based Logical data models
• Record Based Logical data models
• Physical data models
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4. DATA MODELS
Object based
Logical Models
Record based Logical Models
(Semantic)
(Semantic)
ER Model
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OO Model
Relational
Hierarchical
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Network
Physical
Model
Unifying
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5. DATA MODELLING
• A database can be modelled as:
• a collection of entities, and
• relationships among entities
• An entity:
• An object that exists and is distinguishable from other objects
• Example: specific person, company, event, plant
• Entities have attributes
• Example: people have names and addresses
• An entity set
• A set of entities of same type that share the same attributes
• Example: set of all persons, companies, trees, holidays
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6. DATABASE DESIGN PROCESS
• Creating a database involves:
• Design of database schema
• Design of programs that access and update the data
• Design of security scheme to control access to data
Design of
database
schema
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Design of
programs that
access and
update the data
Sumit Mittu, Assistant Professor, CSE/IT
Design of security
scheme to
control access to
data
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7. DATABASE DESIGN PROCESS
• Design Phases
Collect User Requirements
• Fully characterize the needs of prospective
database users and domain experts
Choose Data Model
• Choose a data model and translate above
requirements into a conceptual schema
Specify Functional
Requirements
• Users describe the kinds of operations (or
transactions) to be performed on data
Review Conceptual
Schema
• Designer reviews conceptual schema to ensure
that it meets functional requirements
Physical Implementation
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• Logical design phase
• Physical design phase
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8. DATABASE DESIGN PROCESS
• Pitfalls to avoid during database design process
Incompletenes
s
Redundancy
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Pitfalls
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9. SCRATCH YOUR MIND!!!
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10. THE ENTITY–RELATIONSHIP MODEL
• The E-R Model
• Perceives the real world as consisting of basic objects
(called entities) and relationships among them
• Represents overall logical design of the database
• A semantic model
• Useful in mapping the meanings and interactions of real
world enterprises onto conceptual schema
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11. THE ENTITY–RELATIONSHIP MODEL
• The E-R Model
• Three basic concepts of E-R model
• Entity Sets
• Composed of entities
• Relationship Sets
• Composed of relationships
• Attributes
• Properties of entities (entity sets) and relationships (relationship sets)
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12. THE ENTITY–RELATIONSHIP MODEL
• Entity
• A thing of an object in real world distinguishable from the
others
• E.g. each employee in an enterprise, each notebook, each wall
• Entity may be: concrete or abstract
• Entity Set
• Collection of entities with same set of properties/attributes
• E.g. Set of all employees in an enterprise, set all notebooks in a bag,
set of all walls in a building
• Individual entities that constitute a set are known as extension
of the entity set.
• Two or more entity sets may be disjoint (overlapping)
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13. THE ENTITY–RELATIONSHIP MODEL
(Entities)
(Entity set)
(Entity set)
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14. THE ENTITY–RELATIONSHIP MODEL
• Attributes
• Each entity posses some properties whose values may help
identify an entity uniquely over a set of similar entities
• E.g. name/address of an employee, no. of pages in notebook,
height of a wall
• Each entity has a value for each of its attributes
• Domain or value-set of an attribute is the set of permissible
values for a given attribute for any entity.
• E.g. Age can be a positive integer above 18 for a voter entity.
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15. Types of Attributes
THE ENTITY–RELATIONSHIP MODEL
Simple
Username, Password
Composite
Name composed of first name and last name
Single-valued
Registration No., Password
Multi-valued
Contact No., Subjects
Derived
Age derived from DoB
Null-valued
Criminal Record (that may not exist for some records)
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16. THE ENTITY–RELATIONSHIP MODEL
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17. THE ENTITY–RELATIONSHIP MODEL
• Relationship
• An association among several entities
• Relationship Set
• Set of relationships of same type
• Mathematically, it is a set R of ordered pairs set on n≥2
(possibly non-distinct) entity sets E1, E2, … En
• R = { (e1,e2,…en} | e1 є E1, e2 є E2, … en є En}
• Each ordered pair (e1,e2, … en) is a relationship
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18. THE ENTITY–RELATIONSHIP MODEL
Each line above implies a relationship between entity
from instructor entity set and that from student entity set
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19. THE ENTITY–RELATIONSHIP MODEL
• Relationship Set
• Participation
• The association between entity sets in a relationship
• Types: Total or Partial
• Relationship instance
• An association between named entities
• Entity’s Role
• Function that entity plays in a relationship
• Types: Implicit or Explicit
• Recursive Relationship Set
• Entities of an entity set form relationship with other entities of same
entity set
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20. THE ENTITY–RELATIONSHIP MODEL
• Relationship
• Types (or degree of relationship):
• Binary (degree 2)
• Instructor, Student
• Ternary (degree 3)
• Instructor, Student, Course
• N-ary (degree N)
• Involving entities from N entity sets
• Descriptive attributes
• Attributes assigned to relationships
• Session Year attribute of Instructor-Student relationship
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21. THE ENTITY–RELATIONSHIP MODEL
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22. SCRATCH YOUR MIND!!!
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23. CONSTRAINTS
• Mapping Cardinalities
• or, Cardinality ratios express the number of entities to which
another entity may be associated via a relationship set.
• Types
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One to one
One to many
Many to one
Many to many
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1:1
1:N
N:1
M:N
Husbands :: Wives
Fathers :: Children
Children :: Mothers
Instructors :: Students
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24. CONSTRAINTS
1:N
1:1
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25. CONSTRAINTS
M:N
N:1
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26. CONSTRAINTS
• Participation Constraints
• Total
• When all entities of an entity
participate in relationship set
• E.g. Entity Set A (in diagram)
• Partial
• When only some of the entities
participate in the relationship
set
• E.g. Entity Set B (in diagram)
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27. KEYS
• Keys
• Help identify a set of attributes that suffice to distinguish
entities from each other
• Entity Set Keys
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Super keys
Candidate Keys
Primary Key
Alternate Keys
• Relationship Set Keys
• PK(E1) u PK(E2) u … u PK(En) u {a1,a2,…am}
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28. REMOVING REDUNDANT ATTRIBUTES
• Database design using E-R Model
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Identify the entity sets
Choose appropriate attributes
Form the relationship sets
Eliminate redundant attributes that may
• Exist in multiple entity sets
• E.g. instructor_id, instructor_dept_id and instructor_dept_name
repeated in instructor and student entity. The instructor_dept_id and
instructor_dept_name may be eliminated from student entity set
• Exist even when not actually required
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29. SCRATCH YOUR MIND!!!
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30. THE E-R DIAGRAMS
• E-R Diagram
• Graphical representation of the overall logical structure of
the database
• Basic Structure
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Rectangles, Ellipses, Diamonds,
Lines, Dashed lines, Arrows
Double Lines, Double Diamonds, Double Ellipses
Link Labels (for cardinality and roles)
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31. THE E-R DIAGRAMS
• E-R Diagram
• Weak entity sets
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Identifying or owner entity set
Existence dependence
Identifying relationship
Discriminator attribute
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36. THE E-R DIAGRAMS
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37. THE E-R DIAGRAMS
• E-R Diagram Reduction to Relational Schema
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Representation of Strong Entity Sets with simple attributes
Representation of Strong Entity Sets with complex attributes
Representation of Weak Entity Sets
Representation of Relationship Sets
Redundancy of Schemas
Combination of Schemas
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38. THE E-R DIAGRAMS
• E-R Diagram Design Issues
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Use of entity sets v/s Attributes
Use of Entity Sets v/s Relationship Sets
Binary v/s n-ary relationships
Placement of Relationship Attributes
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