Oracle PL/SQL Interactive Workbook, 2/e

Test Your Thinking Solutions Chapter 9 Introduction to Cursors

  1. Write a nested cursor where the parent cursor calls information about each section of a course. The child cursor counts the enrollment. The only output is one line for each course with the course name, section number, and the total enrollment.

    Answer: Your answer should look similar to the following:

       CURSOR c_course IS
          SELECT course_no, description
            FROM course
           WHERE course_no < 120;
       CURSOR c_enrollment(p_course_no IN course.course_no%TYPE)
          SELECT s.section_no section_no, count(*) count
            FROM section s, enrollment e  
           WHERE s.course_no = p_course_no
             AND s.section_id = e.section_id
           GROUP BY s.section_no;
       FOR r_course IN c_course LOOP
             (r_course.course_no||' '|| r_course.description);
          FOR r_enroll IN c_enrollment(r_course.course_no) LOOP
               (Chr(9)||'Section:  '||r_enroll.section_no||
                ' has an enrollment of: '||r_enroll.count);
          END LOOP;
       END LOOP;
  2. Write an anonymous PL/SQL block that finds all the courses that have at least one section that is at its maximum enrollment. If there are no courses that meet that criterion, then pick two courses and create that situation for each.

    1. For each of those courses, add another section. The instructor for the new section should be taken from the existing records in the instruct table. Use the instructor who is signed up to teach the least number of courses. Handle the fact that, during the execution of your program, the instructor teaching the most courses may change.
    2. Use an exception handling techniques you think are useful to capture error conditions.

    Answer: Your answer should look similar to the following:

       v_instid_min instructor.instructor_id%TYPE;
       v_section_id_new section.section_id%TYPE;
       v_snumber_recent section.section_no%TYPE := 0;
       -- This cursor determines the courses that have at least
       -- one section filled to capacity.
       CURSOR c_filled IS
          SELECT DISTINCT s.course_no
            FROM section s
           WHERE s.capacity = (SELECT COUNT(section_id)
                                 FROM enrollment e
                                WHERE e.section_id = s.section_id);
       FOR r_filled IN c_filled LOOP
          -- For each course in this list, add another section.
          -- First, determine the instructor who is teaching the least
          -- number of courses. If there are more than one instructor
          -- teaching the same number of minimum courses (e.g. if there are 
          -- three instructors teaching 1 course) use any of those
          -- instructors.
          SELECT instructor_id
            INTO v_instid_min
            FROM instructor
                           FROM section
                         WHERE section.instructor_id = instructor.instructor_id
                         GROUP BY instructor_id
                         HAVING COUNT(*) = (SELECT MIN(COUNT(*))
                                              FROM section
                                             WHERE instructor_id IS NOT NULL
                                            GROUP BY instructor_id)
             AND ROWNUM = 1;
          -- Determine the section_id for the new section
          -- Note that this method would not work in a multi-user
          -- environment. A sequence should be used instead.
          SELECT MAX(section_id) + 1
            INTO v_section_id_new
            FROM section;
          -- Determine the section number for the new section
          -- This only needs to be done in the real world if the
          -- system specification calls for a sequence in a parent.
          -- The sequence in parent here refers to the section_no
          -- incrementing within the course_no, and not the section_no 
          -- incrementing within the section_id.
             CURSOR c_snumber_in_parent IS
                SELECT section_no
                FROM section
               WHERE course_no = r_filled.course_no
             ORDER BY section_no;
             -- Go from the lowest to the highest section_no and find
             -- any gaps. If there are no gaps make the new section_no 
             -- equal to the highest current section_no + 1.
             FOR r_snumber_in_parent IN c_snumber_in_parent LOOP
               EXIT WHEN r_snumber_in_parent.section_no > v_snumber_recent + 1;
               v_snumber_recent := r_snumber_in_parent.section_no + 1;
             END LOOP;
             -- At this point, v_snumber_recent will be equal either
             -- to the value preceeding the gap or to the highest
             -- section_no for that course.
          -- Do the insert.
          INSERT INTO section
            (section_id, course_no, section_no, instructor_id)
            (v_section_id_new, r_filled.course_no, v_snumber_recent, 
          -- COMMIT;
       END LOOP;
          DBMS_OUTPUT.PUT_LINE ('An error has occurred');

Select a Chapter for Test Your Thinking Solutions

  1. Programming Concepts
  2. PLSQL Concepts
  3. General Programming Language Fundamentals
  4. SQL in PLSQL
  5. Conditional Control: IF Statements
  6. Conditional Control: CASE Statements
  7. Error Handling and Built-In Exceptions
  8. Iterative Control
  9. Introduction to Cursors
  10. Exceptions
  11. Exceptions: Advanced Concepts
  12. Procedures
  13. Functions
  14. Packages
  15. Advanced Cursors
  16. Stored Code
  17. Triggers
  18. Collections
  19. Records