1Chapter 4

Chapter 4:The Enhanced ER Model and

Business Rules

2Chapter 4

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

4Chapter 4

Different modeling tools may have different notation for the same modeling constructs

b) Microsoft

Visio Notation

Figure 4-1 Basic notation for supertype/subtype notation (cont.)

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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

7Chapter 4

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

8Chapter 4

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

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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

10Chapter 4

Figure 4-4 Example of generalization (cont.)

So we put the shared attributes in a supertype

Note: no subtype for motorcycle, since it has no unique attributes

b) Generalization to VEHICLE supertype

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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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b) Specialization to MANUFACTURED PART and PURCHASED PART

Note: multivalued attribute was replaced by an associative entity relationship to another entity

Created 2 subtypes

Figure 4-5 Example of specialization (cont.)

13Chapter 4

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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b) Partial specialization rule

A vehicle could be a car, a truck, or neither

Figure 4-6 Examples of completeness constraints (cont.)

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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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a) Disjoint rule

Figure 4-7 Examples of disjointness constraints

A patient can either be outpatient or resident, but not both

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b) Overlap rule

A part may be both purchased and manufactured

Figure 4-7 Examples of disjointness constraints (cont.)

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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

27Chapter 4

Packaged data models provide generic models that can be customized for a particular organization’s business rules

28Chapter 4

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

29Chapter 4

DerivationDerived 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

32Chapter 4

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 Action assertions identify corresponding objects that constrain the ability to perform actions on anchor objectsconstrain 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

Action assertion

Anchor object

Corresponding object

Corresponding object

In this case, the action assertion

is a RRestriction

35Chapter 4

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

Action assertionAnchor object

Corresponding object

In this case, the action assertion is an

UUpper LIMLIMit