1. CHAPTER 5 EXCEPTION HANDLING

2. PL/SQL BLOCK STRUCTURE
PL/SQL block has the following structure:
DECLARE
Declaration statements
BEGIN
Executable statements
EXCEPTION
Exception-handling statements
END ;

3. EXCEPTION-HANDLING SECTION
The exception-handling section is the last section of the PL/SQL block.
This section contains statements that are executed when a runtime error occurs within a block.
Runtime errors occur while the program is running and cannot be detected by the PL/SQL compiler.
EXCEPTION
WHEN NO_DATA_FOUND THEN
DBMS_OUTPUT.PUT_LINE
(‘ There is no student with student id 123 ’);
END;

4. Types of exceptions System exceptions User-defined exceptions Named
Unnamed
User-defined exceptions

5. Named system exceptions
Oracle can handle:
CURSOR_ALREADY_OPENED (sqlcode = -6511)
DUP_VAL_ON_INDEX (sqlcode = -0001)
INVALID_CURSOR (sqlcode = -1001)
INVALID_NUMBER (sqlcode = -1722)
LOGIN_DENIED (sqlcode = -1017)
NO_DATA_FOUND (sqlcode = -1403)
TOO_MANY_ROWS (sqlcode = -1422)
…etc…
These are named in the ‘standard’ package in pl/sql.

6. To handle these exceptions explicitly:
These exception names do not need to be declared.
To handle them explicitly, put a clause in the exception section:
EXCEPTION
When DUP_VAL_ON_INDEX Then
dbms_output.put_line(‘record already there’);
END;

7. Example DECLARE v_num1 integer := &sv_num1;
v_result number;
BEGIN
v_result := v_num1 / v_num2;
DBMS_OUTPUT.PUT_LINE (‘v_result: ’|| v_result);
EXCEPTION
WHEN ZERO_DIVIDE THEN
DBMS_OUTPUT.PUT_LINE
(‘A number cannot be divided by zero.’);
END;

8. EXAMPLE DECLARE v_student_id char(5) := &sv_student_id;
v_first_name VARCHAR2(35);
v_last_name VARCHAR2(35);
BEGIN
SELECT first, last
INTO v_first_name, v_last_name
FROM student
WHERE studentid = v_student_id;
DBMS_OUTPUT.PUT_LINE
('Student name: '||v_first_name||' '||v_last_name);
EXCEPTION
WHEN NO_DATA_FOUND THEN
DBMS_OUTPUT.PUT_LINE('There is no such student');
END;

9. HANDLING DIFFERENT EXCEPTIONS
So far, you have seen examples of the programs able to handle a single exception only.
For example, a PL/SQL contains an exception- handler with a single exception ZERO_DIVIDE.
However, many times in the PL/SQL block you need to handle different exceptions.
Moreover, often you need to specify different actions that must be taken when a particular exception is raised.

10. DECLARE v_student_id NUMBER := &sv_student_id; v_enrolled VARCHAR2(3) := 'NO'; BEGIN DBMS_OUTPUT.PUT_LINE (‘Check if the student is enrolled’); SELECT ‘YES’ INTO v_enrolled FROM enrollment WHERE student_id = v_student_id; (‘The student is enrolled into one course’); EXCEPTION WHEN NO_DATA_FOUND THEN DBMS_OUTPUT.PUT_LINE(‘The student is not enrolled’); WHEN TOO_MANY_ROWS THEN (‘The student is enrolled into many courses’); END;
This example contains two exceptions in the single exception handling section.
The first exception, NO_DATA_FOUND, will be raised if there are no records in the ENROLLMENT table for a particular student.
The second exception, TOO_MANY_ROWS, will be raised if a particular student is enrolled into more than one course.

11. OTHERS Handler
You have seen examples of exception-handling sections that have particular exceptions, such as NO_DATA_FOUND or ZERO_DIVIDE.
However, you cannot always predict beforehand what exception might be raised by your PL/SQL block.
In cases like this, there is a special exception handler called OTHERS.
All predefined Oracle errors (exceptions) can be handled with the help of the OTHERS handler.

12. When run, this example produces the following output:
DECLARE
v_instructor_id NUMBER := &sv_instructor_id;
v_instructor_name VARCHAR2(50);
BEGIN
SELECT first_name||' '||last_name
INTO v_instructor_name
FROM instructor
WHERE instructor_id = v_instructor_id;
DBMS_OUTPUT.PUT_LINE
(‘Instructor name is’ ||v_instructor_name);
EXCEPTION
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE(‘An error has occurred’);
END;
When run, this example produces the following output:
Enter value for sv_instructor_id: 100
old 2: v_instructor_id NUMBER := &sv_instructor_id;
new 2: v_instructor_id NUMBER := 100;
An error has occurred
PL/SQL procedure successfully completed.
This demonstrates not only the use of the OTHERS exception handler, but also a bad programming practice.
The exception OTHERS has been raised because there is no record in the INSTRUCTOR table for instructor ID 100.

13. Unnamed system exceptions
These errors are not pre-named, but have a number.
They will be raised automatically by the RDBMS.
The EXCEPTION section handles them in the WHEN OTHERS clause.
To name an unnamed error in your application:
Give the error a name using a PRAGMA, or compiler directive, called EXCEPTION_INIT.
PL/SQL or RDBMS raise the error automatically.
Handle the error in a specially written WHEN clause in the exception section.

14. Unnamed system exceptions
We’ve all seen errors that Oracle throws at us:
ERROR: ORA=12170: TNS: Connect timeout occurred
TNS Listener does not currently know of service requested in connect descriptor
Note:
All of these errors have an error number:
e.g. ORA = means that the connection timeout occurred.
These errors are RAISED automatically by the system, because they are system errors.

15. Example DECLARE deadlock_detected EXCEPTION;
PRAGMA EXCEPTION_INIT(deadlock_detected, -60);
BEGIN
...
-- Some operation that causes an ORA-00060
-- error (see locking)
EXCEPTION
WHEN deadlock_detected THEN
-- handle the error
END;

16. User Defined Exceptions
This type of exception is called a user-defined exception because it is defined by the programmer.
Before the exception can be used, it must be declared.
A user-defined exception is declared in the declaration part of a PL/SQL block as shown below:
DECLARE
exception_name EXCEPTION;
Once an exception has been declared, the executable statements associated with this exception are specified in the exception-handling section of the block.
The format of the exception-handling section is the same as for named exceptions.

17. Example DECLARE e_invalid_id EXCEPTION; BEGIN … EXCEPTION
WHEN e_invalid_id THEN
DBMS_OUTPUT.PUT_LINE ('An id cannot be negative');
END;

18. Raising Exception A user-defined exception must be raised explicitly.
In other words, you need to specify in your program under which circumstances an exception must be raised as shown :
DECLARE
exception_name EXCEPTION;
BEGIN …
IF CONDITION THEN
RAISE exception_name;
ELSE
END IF;
EXCEPTION
WHEN exception_name
THEN
ERROR-PROCESSING
STATEMENTS;
END;