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Chapter 4: The Enhanced ER Model and Business Rules

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The Enhanced ER Model and Business Rules Supertypes and Subtypes Subtype: A subgrouping of the entities in an entity type that has attributes distinct from those in ... – PowerPoint PPT presentation

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Title: Chapter 4: The Enhanced ER Model and Business Rules


1
Chapter 4 The Enhanced ER Model and Business
Rules
2
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

3
Figure 4-1 Basic notation for supertype/subtype
notation
a) EER notation
4
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
5
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
6
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

7
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
8
Generalization and Specialization
  • Generalization The process of defining a more
    general entity type from a set of more
    specialized entity types. BOTTOM-UP
  • Specialization The process of defining one or
    more subtypes of the supertype and forming
    supertype/subtype relationships. TOP-DOWN

9
Figure 4-4 Example of generalization
a) Three entity types CAR, TRUCK, and MOTORCYCLE
All these types of vehicles have common attributes
10
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
11
Figure 4-5 Example of specialization
a) Entity type PART
12
Figure 4-5 Example of specialization (cont.)
b) Specialization to MANUFACTURED PART and
PURCHASED PART
Created 2 subtypes
13
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)

14
Figure 4-6 Examples of completeness constraints
a) Total specialization rule
15
Figure 4-6 Examples of completeness constraints
(cont.)
b) Partial specialization rule
16
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

17
Figure 4-7 Examples of disjointness constraints
a) Disjoint rule
18
Figure 4-7 Examples of disjointness constraints
(cont.)
b) Overlap rule
19
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

20
Figure 4-8 Introducing a subtype discriminator
(disjoint rule)
21
Figure 4-9 Subtype discriminator (overlap rule)
22
Figure 4-10 Example of supertype/subtype
hierarchy
23
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

24
Figure 4-13a Possible entity clusters for Pine
Valley Furniture in Microsoft Visio
Related groups of entities could become clusters
25
Figure 4-13b EER diagram of PVF entity clusters
More readable, isnt it?
26
Figure 4-14 Manufacturing entity cluster
Detail for a single cluster
27
Packaged data models provide generic models that
can be customized for a particular organizations
business rules
28
Business rules
  • Statements that define or constrain some aspect
    of the business
  • Classification of business rules
  • Derivationrule derived from other knowledge,
    often in the form of a formula using attribute
    values
  • Structural assertionrule expressing static
    structure. Includes attributes, relationships,
    and definitions
  • Action assertionrule expressing
    constraints/control of organizational actions

29
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

30
Types of Action Assertions
  • Results from assertion
  • ConditionIF/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 constraintmust 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.')
  • Authorizationprivilege 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.)

31
  • 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
  • Controllingsomething must or must not happen
  • Influencingguideline for which a notification
    must occur

32
Stating an Action Assertion
  • Anchor Objectan object on which actions are
    limited
  • Actioncreation, deletion, update, or read
  • Corresponding Objectsan 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
33
Figure 4-19 Data model segment for class
scheduling
34
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
Action assertion
Anchor object
35
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 Upper
LIMit
Corresponding object
Action assertion
Anchor object
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