1. General Constraints and Event Condition Action Model
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2. Objectives To develop an understanding of integrity constraints.
To introduce the Event-Condition-Action Model
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3. Fundamental Constraints
The  following are basic integrity constraints which must be considered for a relational schema.
Domains
(Not) Null
Participation conditions
Primary keys
Referential Integrity
(these constraints are not discrete and are closely related to each other)
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4. General Constraints
In addition there are typically a large number of other constraints.
They are often called business rules, or general constraints.
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5. e.g. in the Flights example
The departure time at a stopover must be later than the arrival time at the stopover.
 Each stopover arrival time must be later than the departure time at the previous stopover in the flight. 
The departure time of a flight must be later than the arrival time of a flight. 
Each stopover airport must be different from any other airport in the flight
In the Doctors example-
Dates_of_issue and consultation_dates for any patient must be greater than the date_of_birth.
Any doctor must not issue more than 36 prescriptions in one day without authorization.
 There must be a minimum of five minutes between each prescription issue.
  At least one consultation must be with the patient’s registered doctor.
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6. Event-Condition-Action (ECA) Model
Event-database operation, external signal, recursive composition of other events.
Condition-query issued on database
Also Rules- When an event occurs evaluate all the rules that refer to this event. For each relevant rule evaluate the condition. If the condition is satisfied execute action.
Action-user defined program
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7. Example:
A transaction attempts to insert details of a flight that returns to a previous stopover airport-EVENT
Each stopover airport must be different from any other airport in the flight –RULE/CONSTRAINT/CONDITION
Reject insert operation, rollback transaction, raise error/exception details- ACTION
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8. Problems
The model cannot predict 'transitive closure' of events as the logic of action is hidden in application programs.
All consequences of an update cannot be predicted because the update itself may cause event(s) which trigger further actions…..which generate events that trigger actions etc….
There could be contradictory rules where there are large numbers and/or cpomplex rules.
And the mutual effects of rules may not be predicted.
However, this is the common model for implementation of constraints.
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9. Summary
There are a large number of integrity constraints of different types in any domain of discourse
A large proportion of these are ‘general constraints’ not expressed by the data model
The implementation mechanism for ‘general constraints’ normally uses the ECA model
1/12/2019
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