Chapter 10 Exceptions


1)

Create the following script. For each section determine the number of students registered. If this number is equal to or greater than 15, raise the user -defined exception e_too_many_students and display the error message. Otherwise, display how many students are in a section. Make sure that your program is able to process all sections.

A1:

Answer: Your answer should look similar to the following:

 SET SERVEROUTPUT ON SIZE 5000 DECLARE CURSOR section_cur IS SELECT section_id FROM section; v_total NUMBER; e_too_many_students EXCEPTION; BEGIN FOR section_rec in section_cur LOOP BEGIN -- calculate number of students enrolled SELECT COUNT(*) INTO v_total FROM enrollment WHERE section_id = section_rec.section_id; IF v_total >= 15 THEN RAISE e_too_many_students; ELSE DBMS_OUTPUT.PUT_LINE ('There are 'v_total ' students for section ID ' section_rec.section_id); END IF; EXCEPTION WHEN e_too_many_students THEN DBMS_OUTPUT.PUT_LINE ('There are too many ' students for 'section_rec.section_id); END; END LOOP; END; 

In this script, you declare a cursor on the SECTION table. Next, for each section ID returned by the cursor, the number of students enrolled in a given section is computed. If this number equals to or greater than 15, the user-defined exception E_TOO_MANY_STUDENTS is raised. Otherwise, the message specifying how many students are enrolled in a given section is displayed.

When run, this exercise produces the following output (due to the size of the output, only a part of it is shown here):

 
  There are 0 students for section ID 79   There are 1 students for section ID 80   There are 3 students for section ID 81   There are 2 students for section ID 82   There are 2 students for section ID 83   There are 2 students for section ID 84   There are 5 students for section ID 85   There are 6 students for section ID 86   There are 7 students for section ID 87   There are 5 students for section ID 88   There are 12 students for section ID 89     There are 5 students for section ID 155   There are 8 students for section ID 156   PL/SQL procedure successfully completed.  
2)

Modify the script you created in the previous exercise. Once the exception e_too_many_students has been raised in the inner block, re-raise it in the outer block.

A2:

Answer: Your answer should look similar to the following. All changes are shown in bold letters .

 SET SERVEROUTPUT ON SIZE 5000 DECLARE CURSOR section_cur IS SELECT section_id FROM section; v_total NUMBER; e_too_many_students EXCEPTION; BEGIN FOR section_rec in section_cur LOOP BEGIN -- calculate number of students enrolled SELECT COUNT(*) INTO v_total FROM enrollment WHERE section_id = section_rec.section_id; IF v_total >= 15 THEN RAISE e_too_many_students; ELSE DBMS_OUTPUT.PUT_LINE ('There are 'v_total ' students for 'section ID ' section_rec.section_id); END IF; EXCEPTION WHEN e_too_many_students THEN  RAISE;  END; END LOOP;  EXCEPTION   WHEN e_too_many_students THEN   DBMS_OUTPUT.PUT_LINE ('There are too many students.');  END; 

In this exercise, the exception section of the inner has been modified. A DBMS_OUTPUT.PUT_LINE statement has been substituted with the RAISE statement. In addition, an exception section has been added to the outer block. As a result, when an exception is raised in the inner block, it propagates to the outer block, and the cursor loop terminates.

It is important to note that an error message displayed by the DBMS_OUTPUT.PUT_LINE statement must be changed when a E_TOO_MANY_STUDENTS exception is raised in the outer block. In the previous version of this exercise the error message

 
  ('There are too many students for 'section_rec.section_id);  

was placed inside the body of the cursor FOR loop. If the same error message is placed outside the body of the cursor FOR loop, the following error is generated at runtime:

 
  section_rec.section_id);   *   ERROR at line 31:   ORA-06550: line 31, column 10:   PLS-00201: identifier 'SECTION_REC.SECTION_ID' must be declared   ORA-06550: line 30, column 7:   PL/SQL: Statement ignored  

Why do you think this error is generated?



Oracle PL[s]SQL by Example
Oracle PL[s]SQL by Example
ISBN: 3642256902
EAN: N/A
Year: 2003
Pages: 289

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