Chapter 17 Triggers

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	Oracle® PL/SQL® Interactive Workbook, Second Edition By Benjamin Rosenzweig, Elena Silvestrova
	Table of Contents
	Appendix D.  Answers to Test Your Thinking Sections

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1)	Create the following trigger: Create or modify a trigger on the ENROLLMENT table that fires before an INSERT statement. Make sure all columns that have NOT NULL and foreign key constraints defined on them are populated with their proper values.
A1:	Answer: Your trigger should look similar to the following: CREATE OR REPLACE TRIGGER enrollment_bi BEFORE INSERT ON ENROLLMENT FOR EACH ROW DECLARE v_valid NUMBER := 0; BEGIN SELECT COUNT(*) INTO v_valid FROM student WHERE student_id = :NEW.STUDENT_ID; IF v_valid = 0 THEN RAISE_APPLICATION_ERROR (-20000, 'This is not a valid student'); END IF; SELECT COUNT(*) INTO v_valid FROM section WHERE section_id = :NEW.SECTION_ID; IF v_valid = 0 THEN RAISE_APPLICATION_ERROR (-20001, 'This is not a valid section'); END IF; :NEW.ENROLL_DATE := SYSDATE; :NEW.CREATED_BY := USER; :NEW.CREATED_DATE := SYSDATE; :NEW.MODIFIED_BY := USER; :NEW.MODIFIED_DATE := SYSDATE; END; Consider this trigger. It fires before the INSERT statement on the ENROLLMENT table. First, you validate new values for student ID and section ID. If one of the IDs is invalid, the exception is raised and the trigger is terminated. As a result, the INSERT statement causes an error. If both student and section IDs are found in the STUDENT and SECTION tables, respectively, the ENROLL_DATE, CREATED_ DATE, and MODIFIED_DATE are populated with current date, and columns CREATED_BY and MODIFIED_BY are populated with current user name. Consider the following INSERT statement: INSERT INTO enrollment (student_id, section_id) VALUES (777, 123); The value 777, in this INSERT statement does not exist in the STUDENT table and therefore is invalid. As a result, this INSERT statement causes the following error: INSERT INTO enrollment (student_id, section_id) * ERROR at line 1: ORA-20000: This is not a valid student ORA-06512: at "STUDENT.ENROLLMENT_BI", line 10 ORA-04088: error during execution of trigger 'STUDENT.ENROLLMENT_BI'
2)	Create the following trigger: Create or modify a trigger on the SECTION table that fires before an UPDATE statement. Make sure that the trigger validates incoming values so that there are no constraint violation errors.
A2:	Answer: Your trigger should look similar to the following: CREATE OR REPLACE TRIGGER section_bu BEFORE UPDATE ON SECTION FOR EACH ROW DECLARE v_valid NUMBER := 0; BEGIN IF :NEW.INSTRUCTOR_ID IS NOT NULL THEN SELECT COUNT(*) INTO v_valid FROM instructor WHERE instructor_id = :NEW.instructor_ID; IF v_valid = 0 THEN RAISE_APPLICATION_ERROR (-20000, 'This is not a valid instructor'); END IF; END IF; :NEW.CREATED_BY := USER; :NEW.CREATED_DATE := SYSDATE; :NEW.MODIFIED_BY := USER; :NEW.MODIFIED_DATE := SYSDATE; END; This trigger fires before the UPDATE statement on the SECTION table. First, you check if there is a new value for an instructor ID with the help of an IF-THEN statement. If the IF-THEN statement evaluates to TRUE, the instructor's ID is checked against the INSTRUCTOR table. If a new instructor ID does not exist in the INSTRUCTOR table, the exception is raised, and the trigger is terminated. Otherwise, all columns with NOT NULL constraints are populated with their respected values. Consider the following UPDATE statement: UPDATE section SET instructor_id = 220 WHERE section_id = 79; The value 220 in this UPDATE statement does not exist in the INSTRUCTOR table and therefore is invalid. As a result, this UPDATE statement when run causes an error: UPDATE section * ERROR at line 1: ORA-20000: This is not a valid instructor ORA-06512: at "STUDENT.SECTION_BU", line 11 ORA-04088: error during execution of trigger 'STUDENT.SECTION_BU'

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