Transcript The Enhanced ER Model and Business Rules

Chapter 4:
The Enhanced ER Model and
Business Rules
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Supertypes and Subtypes
• Subtype: A subgrouping of the entities in an entity
type that has attributes distinct from those in other
subgroupings
• Supertype: A generic entity type that has a
relationship with one or more subtypes
• Attribute Inheritance:
– Subtype entities inherit values of all attributes of
the supertype
– An instance of a subtype is also an instance of the
supertype
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Figure 4-1 Basic notation for supertype/subtype notation
a) EER
notation
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Figure 4-1 Basic notation for supertype/subtype notation (cont.)
b) Microsoft
Visio
Notation
Different modeling tools may have different notation for the same
modeling constructs
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Figure 4-2 Employee supertype with three subtypes
All employee subtypes
will have emp nbr, name,
address, and date-hired
Each employee subtype
will also have its own
attributes
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Relationships and Subtypes
• Relationships at the supertype level
indicate that all subtypes will participate in
the relationship
• The instances of a subtype may
participate in a relationship unique to that
subtype. In this situation, the relationship
is shown at the subtype level
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Figure 4-3 Supertype/subtype relationships in a hospital
Both
outpatients
and resident
patients are
cared for by
a responsible
physician
Only resident patients are
assigned to a bed
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Generalization and
Specialization
• Generalization: The process of defining
a more general entity type from a set of
more specialized entity types. BOTTOMUP
• Specialization: The process of defining
one or more subtypes of the supertype
and forming supertype/subtype
relationships. TOP-DOWN
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Figure 4-4 Example of generalization
a) Three entity types: CAR, TRUCK, and MOTORCYCLE
All these types of vehicles have common attributes
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Figure 4-4 Example of generalization (cont.)
b) Generalization to VEHICLE supertype
So we put
the shared
attributes in
a supertype
Note: no subtype for motorcycle, since it has no unique attributes
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Figure 4-5 Example of specialization
a) Entity type PART
Only applies to
manufactured parts
Applies only to purchased parts
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Figure 4-5 Example of specialization (cont.)
b) Specialization to MANUFACTURED PART and PURCHASED PART
Created 2
subtypes
Note: multivalued attribute was replaced by an
associative entity relationship to another entity
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Constraints in Supertype/
Completeness Constraint
• Completeness Constraints:
Whether an instance of a supertype
must also be a member of at least one
subtype
– Total Specialization Rule: Yes (double line)
– Partial Specialization Rule: No (single line)
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Figure 4-6 Examples of completeness constraints
a) Total specialization rule
A patient must be either
an outpatient or a
resident patient
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Figure 4-6 Examples of completeness constraints (cont.)
b) Partial specialization rule
A vehicle
could be a
car, a truck,
or neither
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Constraints in Supertype/
Disjointness constraint
• Disjointness Constraints: Whether
an instance of a supertype may
simultaneously be a member of two (or
more) subtypes
– Disjoint Rule: An instance of the supertype
can be only ONE of the subtypes
– Overlap Rule: An instance of the supertype
could be more than one of the subtypes
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Figure 4-7 Examples of disjointness constraints
a) Disjoint rule
A patient can either be outpatient
or resident, but not both
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Figure 4-7 Examples of disjointness constraints (cont.)
b) Overlap rule
A part may be both
purchased and
manufactured
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Constraints in Supertype/
Subtype Discriminators
• Subtype Discriminator: An attribute of the
supertype whose values determine the target
subtype(s)
– Disjoint – a simple attribute with alternative values to
indicate the possible subtypes
– Overlapping – a composite attribute whose subparts
pertain to different subtypes. Each subpart contains a
boolean value to indicate whether or not the instance
belongs to the associated subtype
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Figure 4-8 Introducing a subtype discriminator (disjoint rule)
A simple attribute with
different possible values
indicating the subtype
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Figure 4-9 Subtype discriminator (overlap rule)
A composite
attribute with
sub-attributes
indicating “yes”
or “no” to
determine
whether it is of
each subtype
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Figure 4-10 Example of supertype/subtype hierarchy
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Entity Clusters
• EER diagrams are difficult to read when
there are too many entities and
relationships
• Solution: Group entities and relationships
into entity clusters
• Entity cluster: Set of one or more entity
types and associated relationships
grouped into a single abstract entity type
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Figure 4-13a
Possible entity
clusters for Pine
Valley Furniture in
Microsoft Visio
Related
groups of
entities could
become
clusters
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Figure 4-13b EER diagram of PVF entity clusters
More readable,
isn’t it?
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Figure 4-14 Manufacturing entity cluster
Detail for a single cluster
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Packaged data
models provide
generic models
that can be
customized for a
particular
organization’s
business rules
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Business rules
• Statements that define or constrain some aspect of
the business
• Classification of business rules:
– Derivation–rule derived from other knowledge, often in the
form of a formula using attribute values
– Structural assertion–rule expressing static structure.
Includes attributes, relationships, and definitions
– Action assertion–rule expressing constraints/control of
organizational actions
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Derivation
Derived facts could be:
• "Rental charge is based on base rental price, optional insurances,
and refueling charge."
• "The number of cars (of a group) that will be available the next day
to meet demand is computed as the number of cars of that group
currently in the parking lot, plus the number due in today from
rental." For example, there are 4 group B cars in the parking lot, and
7 are due from rental today, so there should be 11 available to meet
demand for tomorrow."
• "Base rental price for a car is the rate for the group that car's model
belongs to."
• "Number of rentals, turnover and profit of a branch in the past year
can determine the targets for that branch for the next quarter."
A derivation used to derive this derived fact would be:
• Rental charge = Base rental price + Optional insurances + Refueling
charge
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Types of Action Assertions
• Results from assertion
–
–
–
Condition–IF/THEN rule (A condition is an assertion that if something is true, another
business rule will apply. It can be thought of as a 'test' -- if true, it may be the basis for
enforcing or testing other action assertions. For example, a condition can ask: "did a
customer not show a valid driver's license?" "is a customer in arrears?" or "has a
customer placed an order?")
Integrity constraint–must always be true (An integrity constraint is an assertion that
must always be true. It is considered to have immediate enforcement power because it
prohibits any actions that would result in a false truth value. While a condition can test
for a value (e.g., ask "is a car registered?") and then specify some action based on that
test, an integrity constraint can declare that 'a car must be registered' and prohibit any
action that would result in violation of that end state. Such an integrity constraint, for
example, would prohibit both creating a new car instance without a registration value,
as well as setting an existing car's registration to 'null.')
Authorization–privilege statement (An authorization defines a specific prerogative or
privilege with respect to one or more constructs. It is an assertion represented by the
predicate
(Only) x may do y,
where x typically is a user and y is an action that may be executed or performed.
Authorizations are given only to types capable of independent activity (e.g., people,
departments, computers, etc.). For example, only a branch manager of the 'losing'
branch may assign a car for transfer to another branch.)
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•
Forms of assertion
– Enabler–(if true) An enabler is a type of action assertion that, if true, permits or
leads to the existence of the correspondent object. The assertion is true if the
anchor object exists. This has varying interpretations depending on the nature of
the correspondent object:
– Timer–(when true) A timer is a type of action assertion that tests, enables (or
disables), or creates (or deletes) if a specified threshold has been satisfied.
•
– An executive is a type of action assertion that requires (causes) the
execution of one or more actions. The following example shows how
these types can be combined in various ways. In the statement "if a
customer is three months in arrears, then repossess the car," the part
that measures (counts down) 'three months in arrears' and requires
action thereafter is a condition of type timer. A second action assertion
'repossess the car' is an integrity constraint of type executive.
Rigors
– Controlling–something must or must not happen
– Influencing–guideline for which a notification must occur
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Stating an Action Assertion
• Anchor Object–an object on which actions
are limited
• Action–creation, deletion, update, or read
• Corresponding Objects–an object
influencing the ability to perform an action
on another business rule
Action assertions identify corresponding objects that
constrain the ability to perform actions on anchor objects
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Figure 4-19 Data model segment for class scheduling
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Figure 4-20 Business Rule 1: For a faculty member to be assigned to
teach a section of a course, the faculty member must be qualified to
teach the course for which that section is scheduled
Corresponding object
In this case, the
action assertion
is a Restriction
Action assertion
Anchor object
Corresponding object
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Figure 4-21 Business Rule 2: For a faculty member to be assigned to
teach a section of a course, the faculty member must not be assigned to
teach a total of more than three course sections
In this case, the
action assertion
is an
Corresponding
object
Upper LIMit
Action assertion
Anchor object
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