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Database System LifestyleSystem Definition
Implementation
Loading or data conversion
Application conversion
Testing and validation
Operation
Monitoring and maintenance
Design
It is never thisstraightforward!...
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Database Design ProcessReal World
Requirements AnalysisDatabase
Requirements
Conceptual Design
ConceptualSchema
Logical Design
Physical Design
LogicalSchema
FunctionalRequirements
Functional Analysis
AccessSpecifications
Application Pgm Design
E-R Modeling
Choice of aDBMS
Data ModelMapping
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Requirements Collection & Analysis
Examples activities: Identification of user groups and application areas Analysis of the operating environment and processing
requirements Interviews
Caveat: Users change their minds Anticipating users’ future desires is difficult On the one hand: Adaptive system design is good. On the other hand: Good performance requires
freezing important system parameters.
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Conceptual Database Design
Conceptual Schema Design: Database structure, semantics, interrelationships, and
constraints. A stable description of the database (anticipating users’
desires). High-level data model may be useful:
Expressiveness Simplicity Minimality Diagrammatic
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Conceptual Database Design
Schema design strategies Top-down: start from abstraction and use successive
refinements. Bottom-up: start from concrete requirements to find
abstractions. Iterative: mixed top-down and bottom-up as
appropriate (or when
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Bottom-Up Design
Schema 1 Schema 2
Correspondences& Conflicts
Correspondences& Conflicts
Modification forConformance
Modification forConformance
Merging
Merging
Integrated Schema
Final Schema
Conflicts:NamingTypeDomainConstraints
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Conceptual Transaction Design
This is the part of application design that impacts database design They indicate the application requirements
Transactions are… units of application programs units of run-time jobs
Types of transactions: read-only (retrieval) update (read and write)
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Choice of a DBMS
Technical Factors: Data model, user interfaces. Programmer interfaces (programming languages),
application development tools. Storage, access methods.
Economic Factors: Software, hardware, database. Acquisition, maintenance, conversion. Personnel, training, operating.
Political Factors: Standards, proprietary v.s. open systems, etc.
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Data Model Mapping
Convert the conceptual and external schema into the DBMS’s high-level data model. The result of this phase is a set of DDL statements in
the language of the chosen DBMS.
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Physical Database Design Storage structures and access paths General user requirements – examples:
Response-time: 95% of transactions must answer within 2 seconds
Space utilization: disk should not be more than half empty Throughput: average number of transactions per second
Separate read-only queries from update transactions Expected frequency of queries and transactions. User requirements on response-time and throughput Optimization techniques:
Denormalization, duplication Indexed files for scan and hashing for random access
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Physical Design Guidelines
Focus on attributes that are used frequently: Choose the primary index carefully Use secondary indices when necessary Reduce indices when updates become frequent Use hashing for equality selection and join
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Conceptual DesignReal World
Requirements AnalysisDatabase
Requirements
Conceptual Design
ConceptualSchema
Logical Design
Physical Design
LogicalSchema
FunctionalRequirements
Functional Analysis
AccessSpecifications
Application Pgm Design
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Top-Down Design Determine the entities, attributes, relationships Model them properly Map the resulting E-R model into a data model
Conceptual No physical details Easier to detect conceptual design errors
One of the logical database design aids Closer to other data models Significant amount of research within the database
community Easy mappings to other data models possible
Entity-Relationship Modeling
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Entity An object that exists in the real world, that has certain
properties and that is distinguishable from other objects Example
Employee Project
Attribute The properties of entities and relationships Example
Employee Employee No, Name, Title, Salary Project Project No, Name, Budget
Relationship Associations between two or more entities Example
Manage Employees manage projects Work Employees work in projects
Entity-Relationship Modeling
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Entity type is an abstraction which defines the properties (attributes) of a similar set of entities Example:
Employee Name, Title, Salary Project Number, Budget, Location
Entity instances are instantiations of types Example:
Employee Joe, Jim, ... Project Compiler design, Accounting, ...
An entity instance can have multiple entity types Example :
If we also want to have an EMPLOYEE entity type, then every engineer is also an employee
Entity class (or entity set) is a set of entity instances that are of the same type
Similar arguments can be made for relationships
Entity Types and Instances
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Types and InstancesWorks OnEmployee
E1 J. Doe
E2 M. Smith
E3 A. Lee
E4 J. Miller
E5 B. Casey
E6 L. Chu
E7 R. Davis
E8 J. Jones
Project
P1 Instrumentation
P2 Database Develop.
P3 CAD/CAM
P4 Maintenance
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Attributes
Describe properties of entities and relationships An instance of an attribute is a value, drawn from
given domain, which presents the set of possible values of the attribute.
Types: Single vs multivalued
Single: Social insurance number Multi: Lecturers of a course
Simple vs composite Composite: Address consisting of Apt#, Street, City, Zip
Stored vs derived attribute Stored: Individual mark of a student Derived: Average mark in a class
Key attribute - identifier
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Entity identifier One or more of the attributes that can uniquely identify
each instance of a given entity type Example
Employee Employee No Project Project No
Relationship identifier A means of identifying each relationship instance. Usually a composite identifier consisting of the
identifiers of the two entity types that it relates Example
Works (Employee No, Project No)
Identifiers
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Entities-Attributes-Relationships
DEPARTMENT
WAREHOUSE PART EMPLOYEE
PROJECTSUPPLIER
LOCATION
SupplierNo
SupplierName
LocationProjectNo
ProjectName
Budget
Wareh.No
Wareh.Name
Location PartNo
PartName
QTY WGT
City Dept.No
Dept.Name
Manager
Emp.No
Emp.Name
Title SalaryAddr
are supplied by supply
are supplied by
supply
are used by
use
work on
employ
manage
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Entity types and instances
Attributes
Relationships
E-R Notation
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E-R Diagrams
EMPLOYEE PROJECT
Responsibility
Duration
Budget
ProjectName
Project NoEmployee No EmployeeName
SalaryTitle
WORKS ON
Address
CityApt. #
Street #
NoEmp
Location
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Semantics of E-R Models
There is a need to capture the semantics of entities and relationships
This is done by means of constraints Primary Key
One of the identifiers of each entity and relationship
Cardinality constraints types of relationships
Existence (participation) constraint Referential integrity
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Fundamental ones One-to-one Many-to-one (one-to-many) Many-to-many
Recursive relationships
There can be multiple relationships between two entity types
Types of Relationships
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Each instance of one entity class E1 can be associated with at most one one instance of another entity class E2 and vice versa.
Example : Each employee can work in at most one project and each
project employs at most one employee.
One-to-One Relationship
EMPLOYEE PROJECT
Responsibility
Duration
Budget
ProjectName
Project NoEmployee No EmployeeName
SalaryTitle
WORKS ON1 1
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One-to-One RelationshipWORKS_ON
Relationship Instances
EMPLOYEE Set PROJECT Set
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Each instance of one entity class E1 can be associated with zero or more instances of another entity class E2, but each instance of E2 can be associated with at most 1 instance of E1.
Example : Each employee can work in at most one project; each
project can employ many engineers.
Many-to-One Relationship
EMPLOYEE PROJECT
Responsibility
Duration
Budget
ProjectName
Project NoEmployee No EmployeeName
SalaryTitle
WORKS ON1N
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Many-to-One RelationshipWORKS_ON
Relationship Instances
EMPLOYEE Set PROJECT Set
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Each instance of one entity class can be associated with many instances of another entity class, and vice versa.
Example : Each employee can work in many projects; each
project can employ many employees
Many-to-Many Relationship
EMPLOYEE PROJECT
Responsibility
Duration
Budget
ProjectName
Project NoEmployee No EmployeeName
SalaryTitle
WORKS ONN M
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Many-to-Many RelationshipWORKS_ON
Relationship Instances
EMPLOYEE Set PROJECT Set
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An entity instance of type T1 is in a relationship with another entity instance of type T1.
It assumes multiple roles.
Recursive Relationships
EMPLOYEE
MANAGES
1 N
Man
ager
Sub
ordi
nate
PART
CONTAIN
M N
Is p
art o
f
Con
sist
s of
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Multiple Relationships
EMPLOYEE PROJECT
ResponsibilityDuration
Budget
ProjectName
Project NoEmployee No EmployeeName
SalaryTitle
WORKS ON
N M
MANAGES
1 1
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A relationship can link more than one type of entity.
Higher-Order Relationships
SUPPLIER PROJECT
ResponsibilityDuration
Budget
ProjectName
Project NoSupplier No SupplierName
LocationCredit
SUPPLY
N M
PROVIDE
N M
PART
LPart No
PartName
Qty
Wgt
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Be careful in defining and interpreting relationships. For example, consider the following diagram.
Can we find, for any given employee, which department he is in?
Conversely, can we find, for a given department, which employees are in that department?
Connection Traps
DIVISION
DEPARTMENT EMPLOYEE
1
N
1
N
INCLUDESDEPT
INCLUDESEMP
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One solution is to change the relationship definition.
Connection Traps
DIVISION
DEPARTMENT
EMPLOYEE
1
N 1
N
INCLUDESDEPT
INCLUDESEMP
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What will happen if some employees are connected with divisions (e.g., as consultants to division heads), but are not included in any department?
Connection Traps
DIVISION
DEPARTMENT
EMPLOYEE
1
N 1
N
INCLUDESDEPT
INCLUDESEMP
CONSULTSFOR NM
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Sometimes it is necessary to simplify some of the relationships Some older data models cannot handle them
Even object models sometimes require relationships to be binary
Some E-R based database design tools permit binary relationships only
Types of simplifications Many-to-many Two one-to-many Higher order relationships binary relationships
Simplification is done by creating new relationships Connection traps cause significant difficulties
Simplifications
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Can not do by simple creation of two 1:N relationships between the two entity classes. N:M relationship indicates that there is no dependence between the
instances of the two entity classes. 1:N relationship forces a dependency.
Consider N:M relationship between EMPLOYEE and PROJECT
Many-to-Many Simplification
EMPLOYEE PROJECTWORKS ONN M
EMPLOYEE PROJECT
WORKS ON N
M EMPLOYS
1
1
WRONG
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EMPLOYEE PROJECTWORKS ONN M
Treat the relationship as an entity class. Define suitable relationships among three entities.
This simplification is not necessary for mapping into the relational model, but is important for mapping into other models.
Many-to-Many Simplification
EMPLOYEE PROJECT
EMP--EMP1
M
EMP--PROJ
EMPLOYMENT M
1
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A relationship can link more than one type of entity.
Higher-Order Relationships
SUPPLIER PROJECT
ResponsibilityDuration
Budget
ProjectName
Project NoSupplier No SupplierName
LocationCredit
SUPPLY
N M
PROVIDE
N M
PART
LPart No
PartName
Qty
Wgt
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Higher-Level RelationshipsCreate an intermediate weak entity type
SUPPLIER PROJECTSUPPLYN M
ORD-PART
N M
PART
L
SUP-ORD ORD-PROJ
11
1
ORDER
For the time being, ignore the double rectangle and double lines
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Specialization
An entity type E1 is a specialization of another entity type E2 if E1 has the same properties of E2 and perhaps even more.
E1 IS-A E2
MANAGER
EMPLOYEE
MANAGER
EMPLOYEE
isaor
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Attribute Inheritance
EMPLOYEE
Employee No EmployeeName
Salary
Title Address
MANAGER
Employee No EmployeeName
Salary
Title Address
Expense Act. Condo
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Specialization
ENGINEER SECRETARY SALESPERSON
EMPLOYEE
Employee No EmployeeName
Salary
Title Address
Project Office Specialty Office CarRegion
isa
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Subclass/Superclass
This is related to instances of entities that are involved in a specialization/generalization relationship
If E1 specializes E2, then each instance of E1 is also an instance of E2. Therefore
Class(E1) Class(E2)
ExampleClass(Manager) Class(Employee)
Class(Employee) = Class(Engineer) Class(Secretary) Class(Salesperson)
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Constraints
Referential integrity When there is a 1:1 or M:1 relationship R between entity
types E1 and E2, if one and exactly one instance of E2 has to exist for a given instance of E1, a referential integrity constraint exists
Participation constraint Determines whether instances of a given entity can exist
without participating in a relationship
Cardinality constraint Relationship types (1:1, M:1, M:N) and their refinement
where the exact number is specified
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Referential Integrity
Assume that for a given project, there has to be one and only one employee managing it
EMPLOYEE PROJECT1 1
MANAGES
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Participation Constraints
Whether or not the existence of an entity depends on its being related to another entity via the relationship type
Total: If entity Ei is in total participation with relation R, then every entity instance of Ei has to participate via relation R to an entity instance of another entity type Ej
Partial: Only some entity instances participate
EMPLOYEE PROJECT1 1
MANAGES
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Specialization Constraints
Disjoint Entity instances in a subclass can not exist in more than
one subclass E.g., an employee can not be a secretary and an
engineer at the same time
Overlapping Entity instances can be members of multiple subclasses E.g., an object can both be manufactured and purchased
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Specialization Constraint Combinations
disjoint, total
disjoint, partial
overlapping, total
overlapping, partial
d
d
o
o
isad
isao
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Total & Partial Disjoint
EMPLOYEE
Employee No EmployeeName
Salary
Title Address
SECRETARYENGINEER
Project Office Specialty Office
SALESPERSON
CarRegion
d
d
HOURLY_EMP
SALARIED_EMP
Hourly Rate
Salary
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Total Overlapping
PART
Part No PartName
Qty
Wgt
o
MANUFACTURED_PART PURCHASED_PART
Batch No Drawing No Price
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Strong entities: The instances of the entity class can exist on their own, without participating in any relationship.
Also called non-obligatory membership.
Weak entities: Each instance of the entity class has to participate in a relationship in order to exist. Keys are imported from dependent entity.
Also called obligatory membership.
Special type of total participation
Strong and Weak Entity Sets
PROJECT
Balance
ExpensesIncome
1
1
RECORDS
BUDGET