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Lets design a trigger from scratch now, based on a simple set of rules that the trigger must enforce. Well create a trigger that prevents a students from enrolling in courses, unless the student satisfies a minimum student level (freshman, sophomore, junior, senior, masters, or doctoral).
While the final functionality of the trigger is a rule of the university, the design of the trigger must be grounded in the triggers associated table and any relationships between the associated table and other tables. For each students classes, a row must exist in the ENROLLED_CLASSES table, which has this structure:
ssn NOT NULL varchar2 (9) course_number NOT NULL number (5) audit_flag varchar2 (1)
The ssn column in the ENROLLED_CLASSES table has a foreign key relationship to the ssn column in the STUDENTS table, as follows:
ssn NOT NULL varchar2 (9) first_name NOT NULL varchar2 (10) last_name NOT NULL varchar2 (12) street_address NOT NULL varchar2 (30) apartment_number NOT NULL varchar2 (4) city NOT NULL varchar2 (30) state_code NOT NULL varchar2 (2) zip_code NOT NULL number (5) home_phone NOT NULL number (10) financing_num NOT NULL integer (9) student_level NOT NULL number (1) degree_plan_code number (5) overall_gpa number (3, 2) most_recent_gpa number (3, 2) middle_name varchar2 (10)
The course_number column in the ENROLLED_CLASSES table has a foreign key relationship with the SCHEDULED_COURSES table, as follows:
course_number NOT NULL number (5) course_credits NOT NULL number (1) course_hours NOT NULL number (1) course_time NOT NULL varchar2 (2) course_location NOT NULL number (5) min_student_level number (1) credit_flag varchar2 (1)
This means that our trigger must retrieve information from two different places (the SCHEDULED_COURSES and the STUDENTS tables) to determine whether the student can enroll in the class.
Now that weve examined the data structures that well have to deal with to implement the trigger, lets move on and start examining how the trigger has to work.
One of the most important steps in designing a trigger is making sure the trigger fires at the right time. This is determined by the way the triggers base table is used in day-to-day operations.
In order for a student to register for a class, a row must be created in the ENROLLED_CLASSES table. Because INSERT statements are used to create rows, the trigger we create must obviously fire when a new row is inserted into the table.
The real question is whether rows that already exist in the ENROLLED_CLASSES table can be updated. It turns out that a student could very well update a row by deciding to take a specific class at a different time or in a different location. This means that the trigger must also fire when an UPDATE statement is executed on the ENROLLED_CLASSES table.
Because each row in the ENROLLED_CLASSES table represents a single class, the trigger must fire for every row in the table.
We now know that the trigger must accomplish the following tasks whenever a new row is created or an existing row is modified in the ENROLLED_CLASSES table:
Based on this definition of what the trigger has to do, we can develop some pseudocode for the trigger that clearly illustrates the logical steps necessary to enforce the business rules. This pseudocode is shown in Listing 7.18.
Listing 7.18 Pseudocode for the ENROLLED_CLASSES_ARIU trigger.
for each row created or updated loop get the student's current level; get the minimum level for the course; if the student's level is less than the course level then raise an error; end if; end loop;
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