Different Kinds of Exception in DBMS

Errors
• Two types of errors can be found in a program:
compilation errors and runtime errors.
• There is a special section in a PL/SQL block that
handles the runtime errors.
• This section is called the exception-handling section,
and in it, runtime errors are referred to as
exceptions.
• The exception-handling section allows programmers
to specify what actions should be taken when a
specific exception occurs.

How to handle Exception
• In order to handle run time errors in the
program, an exception handler must be
added.
• The exception-handling section has the
following structure:
EXCEPTION
WHEN EXCEPTION_NAME
THEN
ERROR-PROCESSING STATEMENTS;
• The exception-handling section is placed
after the executable section of the block.

How exception can be classified???
 Build-in exception
 Userdefined exception

BUILT-IN EXCEPTIONS
 When a built-in exception occurs, it is said to
be raised implicitly.
 In other words, if a program breaks an Oracle
rule, the control is passed to the exception-
handling section of the block.
 At this point, the error processing statements
are executed.
 It is important for you to realize that after the
exception-handling section of the block has
executed, the block terminates.
 Control will not return to the executable section
of this block.

This example produces the
following output:
There is no such student
PL/SQL procedure
successfully completed.
Why???
Because there is no record
in the STUDENT table with
student ID 101, the SELECT
INTO statement does not
return any rows.
As a result, control passes
to the exception-handling
section of the block, and
the error message “There is
no such student” is
displayed on the screen.
Even though there is a
DBMS_OUTPUT.PUT_LINE
statement right after the
SELECT statement, it will
not be executed because
control has been
transferred to the
exception-handling section.
Example
DECLARE
v_student_name
VARCHAR2(50);
BEGIN
SELECT first_name||‘
’||last_name
INTO v_student_name
FROM student
WHERE student_id = 101;
(‘Student name
is’||v_student_name);
EXCEPTION
WHEN NO_DATA_FOUND
THEN
(‘There is no such student’);
END;

What are predefined exception??
 The list shown below explains some commonly used predefined
exceptions and how they are raised:
 NO_DATA_FOUND This exception is raised when a SELECT
INTO statement, which makes no calls to group functions, such
as SUM or COUNT, does not return any rows.
 For example, you issue a SELECT INTO statement against
STUDENT table where student ID equals 101.
 If there is no record in the STUDENT table passing this criteria
(student ID equals 101), the NO_DATA_FOUND exception is
raised.

TOO_MANY_ROWS
 This exception is raised when a SELECT INTO
statement returns more than one row.
 By definition, a SELECT INTO can return only single
row.
 If a SELECT INTO statement returns more than one
row, the definition of the SELECT INTO statement
is violated.
 This causes the TOO_MANY_ROWS exception to be
raised.
 For example, you issue a SELECT INTO statement
against the STUDENT table for a specific zip code.
 There is a big chance that this SELECT statement
will return more than one row because many
students can live in the same zip code area.

ZERO_DIVIDE
 This exception is raised when a division operation is
performed in the program and a divisor is equal to zero.
 Previous example in the illustrates how this exception is
raised.
LOGIN_DENIED
 This exception is raised when a user is trying to login on
to Oracle with invalid username or password.

PROGRAM_ERROR
 This exception is raised when a PL/SQL program
has an internal problem
VALUE_ERROR
 This exception is raised when conversion or size
mismatch error occurs.
 For example, you select student’s last name into a
variable that has been defined as VARCHAR2(5).
 If student’s last name contains more than five
characters, VALUE_ERROR exception is raised.

DUP_VALUE_ON_INDEX
 This exception is raised when a program tries to
store a duplicate value in the column or columns
that have a unique index defined on them.
 For example, you are trying to insert a record into
the SECTION table for the course number “25,”
section 1.
 If a record for the given course and section
numbers already exists in the SECTION table,
DUP_VAL_ON_INDEX exception is raised because
these columns have a unique index defined on
them.

Example
FROM enrollment
WHERE student_id = v_student_id;
('Student is registered for '||v_total||' course(s)');
*/inner block*/EXCEPTION
WHEN VALUE_ERROR OR INVALID_NUMBER
THEN
DBMS_OUTPUT.PUT_LINE('An error has
occurred');
END;
*/outer block */
EXCEPTION
WHEN NO_DATA_FOUND
THEN
DBMS_OUTPUT.PUT_LINE('There is no such student');
END;

• The inner block has structure similar to the outer block.
• It has a SELECT INTO statement and an exception
section to handle errors.
• When VALUE_ERROR or INVALID_NUMBER error occurs
in the inner block, the exception is raised.
• It is important that you realize that exceptions
VALUE_ERROR and INVALID_NUMBER have been
defined for the inner block only.
• Therefore, they can be raised in the inner block only.
• If one of these errors occurs in the outer block, this
program will be unable to terminate successfully.

User Defined Exceptions
• Often in programs you may need to handle
problems that are specific to the program you
write.
• For example, your program asks a user to enter
a value for student_id. This value is then
assigned to the variable v_student_id that is
used later in the program.
• Generally, you want a positive number for an id.
By mistake, the user enters a negative number.
• However, no error has occurred because
student_id has been defined as a number, and
the user has supplied a legitimate numeric
value.
• Therefore, you may want to implement your
own exception to handle this situation.

How to declare user-defined exception
• A user-defined exception is declared in
the declarative 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 built-in
exceptions

Raising Exception
• 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;

example
declare
e exception;
vamt number(10);
begin
select amt into vamt from
ac_det where acno=124;
vamt:=vamt-77000;
if(vamt<2000) then
raise e;
end if;
exception
when e then
dbms_output.put_line('Minimum
balance amount conflict');
* end;
SQL> /
Declare exception if the particular data
is not found
Raise exception ‘E’
If ‘E’ is exception the it will be
displayed as
‘minimum amount conflict’

Different Kinds of Exception in DBMS

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