Transcript weak entity type

Chapter 7
Data Modeling Using the EntityRelationship Model
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High-level conceptual model
Used for the conceptual design of DB
applications
Many DB design tools employs its concepts
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Chen 1976 -- MIT
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“The Entity Relationshipodel -- Toward a Unified View
of Data” ACM - TODS
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Overview of Database Design Process
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Two main activities:
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Focus in this chapter on database design
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Database design
Applications design
To design the conceptual schema for a database
application
Applications design focuses on the programs and
interfaces that access the database
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Generally considered part of software engineering
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Entities, Attributes
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Entity - a ”thing” in the real world with
independent existence
Attribute - properties that describe the entity
 Simple (atomic) vs. composite
 Single-values vs. multivalued
 Stored vs. derived
 Null values
 Complex Attributes
{AddressPhone({Phone(areacode,phonenumber)},
Address(StreetAddress(Number,Street,AptNumber),City,State,Zip))}
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Entity Types, Entity Set, Keys, and
Value Sets
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Entity Types: Defines a collection (or set) of entities that
have the same attributes
e.g. employee, student
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Entity Set: The collections of all entities of a particular
entity type
e.g. employee = set of all employees
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Value Sets: specifies the set of values assigned to attribute
(domain of values)
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Dependencies
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1) Existence Dependency
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X and Y are entities
Y entity cannot exist without X entity
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Y cannot enter the database unless its corresponding
X is there
if X is dropped from the database, Y must be dropped
as well
X = strong, parent, owner, dominant
Y = weak, child, dependent, subordinate
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Weak Entity Types
An entity that does not have a key attribute
 A weak entity must participate in an identifying relationship
type with
an owner or identifying entity type
 Entities are identified by the combination of:
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A partial key of the weak entity type
The particular entity they are related to in the identifying
entity type
Example:
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A DEPENDENT entity is identified by the dependent’s first
name, and the specific EMPLOYEE with whom the dependent is
related
Name of DEPENDENT is the partial key
DEPENDENT is a weak entity type
EMPLOYEE is its identifying entity type via the identifying
relationship type DEPENDENT_OF
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2) Identifier Dependency
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Occurs when the weak entity set does not
have a candidate key, and its instances are
indistinguishable w/o a relationship with other
entity (e.g. grade).
3) Referential Dependency
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Occurs when a weak entity contains a foreign
key of the corresponding strong entity (e.g. essn in
Dependent table)
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Referential Constraint
 A nonnull value of the foreign key attribute in the
entity instance must always equal the value of the
primary key of an associated strong entity instance.
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Many-to-one (N:1) Relationship
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Many-to-many (M:N) Relationship
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Displaying a recursive relationship
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In a recursive relationship type.
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Both participations are same entity type in different
roles.
For example, SUPERVISION relationships between
EMPLOYEE (in role of supervisor or boss) and
(another) EMPLOYEE (in role of subordinate or
worker).
In following figure, first role participation labeled
with 1 and second role participation labeled with
2.
In ER diagram, need to display role names to
distinguish participations.
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A Recursive Relationship Supervision`
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Recursive Relationship Type is: SUPERVISION
(participation role names are shown)
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Notation for Constraints on
Relationships
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Cardinality ratio (of a binary relationship): 1:1,
1:N, N:1, or M:N
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Participation constraint (on each participating
entity type): total (called existence dependency)
or partial.
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Shown by placing appropriate numbers on the
relationship edges.
Total shown by double line, partial by single line.
NOTE: These are easy to specify for Binary
Relationship Types.
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Alternative (min, max) notation for
relationship structural constraints:
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Specified on each participation of an entity type E in a relationship
type R
Specifies that each entity e in E participates in at least min and at
most max relationship instances in R
Default(no constraint): min=0, max=n (signifying no limit)
Must have minmax, min0, max 1
Derived from the knowledge of mini-world constraints
Examples:
 A department has exactly one manager and an employee can
manage at most one department.
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Specify (0,1) for participation of EMPLOYEE in MANAGES
Specify (1,1) for participation of DEPARTMENT in MANAGES
An employee can work for exactly one department but a
department can have any number of employees.
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Specify (1,1) for participation of EMPLOYEE in WORKS_FOR
Specify (0,n) for participation of DEPARTMENT in WORKS_FOR
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The (min,max) notation for relationship
constraints
Read the min,max numbers next to the entity
type and looking away from the entity type
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COMPANY ER Schema Diagram using (min,
max) notation
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Alternative diagrammatic notation
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ER diagrams is one popular example for
displaying database schemas
Many other notations exist in the literature and
in various database design and modeling tools
Appendix A illustrates some of the alternative
notations that have been used
Unified Modeling Language (UML) class
diagrams is representative of another way of
displaying ER concepts that is used in several
commercial design tools
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Summary of notation for ER diagrams
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Unified Modeling Language (UML) class
diagrams
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Represent classes (similar to entity types) as large rounded
boxes with three sections:
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Relationships (called associations) represented as lines
connecting the classes
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Top section includes entity type (class) name
Second section includes attributes
Third section includes class operations (operations are not in
basic ER model)
Other UML terminology also differs from ER terminology
Used in database design and object-oriented software
design
UML has many other types of diagrams for software design
(see Chapter 12)
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UML class diagram for COMPANY
database schema
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Other alternative diagrammatic notations
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Relationships of Higher Degree
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Relationship types of degree 2 are called binary
Relationship types of degree 3 are called ternary
and of degree n are called n-ary
In general, an n-ary relationship is not equivalent
to n binary relationships
Constraints are harder to specify for higherdegree relationships (n > 2) than for binary
relationships
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Discussion of n-ary relationships (n > 2)
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In general, 3 binary relationships can represent different
information than a single ternary relationship (see Figure
3.17a and b on next slide)
If needed, the binary and n-ary relationships can all be
included in the schema design (see Figure 3.17a and b,
where all relationships convey different meanings)
In some cases, a ternary relationship can be represented as
a weak entity if the data model allows a weak entity type to
have multiple identifying relationships (and hence multiple
owner entity types) (see Figure 3.17c)
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Example of a ternary relationship
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Discussion of n-ary relationships (n > 2)
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If a particular binary relationship can be derived
from a higher-degree relationship at all times,
then it is redundant
For example, the TAUGHT_DURING binary
relationship in Figure 3.18 (see next slide) can be
derived from the ternary relationship OFFERS
(based on the meaning of the relationships)
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Another example of a ternary
relationship
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Displaying constraints on higher-degree
relationships
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The (min, max) constraints can be displayed on the edges
– however, they do not fully describe the constraints
Displaying a 1, M, or N indicates additional constraints
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An M or N indicates no constraint
A 1 indicates that an entity can participate in at most one
relationship instance that has a particular combination of the
other participating entities
In general, both (min, max) and 1, M, or N are needed to
describe fully the constraints
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Data Modeling Tools
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A number of popular tools that cover conceptual modeling
and mapping into relational schema design.
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POSITIVES:
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Examples: ERWin, S- Designer (Enterprise Application Suite),
ER- Studio, etc.
Serves as documentation of application requirements, easy
user interface - mostly graphics editor support
NEGATIVES:
Most tools lack a proper distinct notation for relationships with
relationship attributes
 Mostly represent a relational design in a diagrammatic form
rather than a conceptual ER-based design
(See Chapter 12 for details)
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Some of the Currently Available
Automated Database Design Tools
COMPANY
TOOL
FUNCTIONALITY
Embarcadero
Technologies
ER Studio
Database Modeling in ER and IDEF1X
DB Artisan
Database administration, space and security
management
Oracle
Developer 2000/Designer
2000
Database modeling, application development
Popkin
Software
System Architect 2001
Data modeling, object modeling, process
modeling, structured analysis/design
Platinum
(Computer
Associates)
Enterprise Modeling Suite:
Erwin, BPWin, Paradigm
Plus
Data, process, and business component
modeling
Persistence
Inc.
Pwertier
Mapping from O-O to relational model
Rational (IBM) Rational Rose
UML Modeling & application generation in
C++/JAVA
Resolution
Ltd.
Conceptual modeling up to code maintenance
Xcase
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Extended Entity-Relationship (EER)
Model (in next chapter)
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The entity relationship model in its original form
did not support the specialization and
generalization abstractions
Next chapter illustrates how the ER model can be
extended with
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Type-subtype and set-subset relationships
Specialization/Generalization Hierarchies
Notation to display them in EER diagrams
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Chapter Summary
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ER Model Concepts: Entities, attributes,
relationships
Constraints in the ER model
Using ER in step-by-step conceptual schema
design for the COMPANY database
ER Diagrams - Notation
Alternative Notations – UML class diagrams,
others
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Mapping an E-R Model to a
Relational Model
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1) The entity sets
represented by rectangles
becomes relations
represented by tables
2) For strong entity sets,
attributes represented by
ovals become attributes of
the relations or column
headings of the table
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For weak entity set - all
attributes include all the
attributes of the weak
entity, plus the primary
key of the owner entity
3) The table for a
relationship set has
column headings
consisting of the
associated entities, plus
column for descriptive
attributes if any
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Examples
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http://www.meds-sdmm.dfompo.gc.ca/meds/Prog_Nat/NCIS/Definitio
n/Ncis_ER_diagram.gif
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Data Modeling Tools - Visio
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2007
Visio 2007 Demo: Show It Like It Is -- Connect Data to
Your Visio Diagram
This demo shows how you can use Microsoft Office Visio Professional 2007 to analyze
how a process or other area is actually performing by connecting data to the shapes
in your diagram.
1/30/2008
Visio 2007 Demo: Create a background and watermark for your
drawings
Learn how to create a background and watermark for your drawings in
Microsoft Office Visio 2007.
1/25/2008
Visio 2007 Demo: Let Data Tell Its Story with Data Graphics
In this demo, you’ll see how to use data graphics in your Microsoft Office
Visio 2007 diagram to present the right data in the right way and to make
your points clearly.
1/30/2008
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Visio 2007 Demo: Give Your Diagram a Makeover by
Applying a Theme
Use themes in Microsoft Office Visio Professional 2007 to easily add color
and style to your diagram and create an attractive, engaging piece.
1/30/2008
Visio 2007 Demo: Get a New Perspective on Data with
PivotDiagrams
In this demo, you’ll see how PivotDiagrams can help you get a fresh
perspective on your data.
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Oracle Designer – Oracle Designer
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http://www.oracle.com/technology/produc
ts/designer/demos.htm
Database Design and Generation
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Creating Entity Diagrams (Viewlet)
Creating Database Designs (Viewlet)
Working With Server Models (Viewlet)
Generating Databases (Viewlet)
Capturing Database Designs (Viewlet)
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Data Modeling Tools
Sybase – PowerDesigner
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http://www.sybase.com/products/modelingmetadata/powerdesign
er/demonstrationvideos
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http://www.datamodel.org/PowerDesigner/
Database Designers the technical video
demonstrates:
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A Walk Through of a Sample Conceptual Data Model
Reverse Engineer a PDM from a Database
Iterative Bi-level Design with PowerDesigner
Keeping the Modeling and the Servers Synchronized
Download this video:
http://download.sybase.com/presentation/pdvideo/pd_d
ba1.exe
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