Exception Handling

Database Lab
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Predefined & User Defined
Exceptions
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• An exception is an error condition during a program
execution.An error or a warning event is called an exception.
• Any abnormal condition or say event that interrupts the
normal flow of your program’s instructions at run time is an
exception. Or in simple words you can say an exception is a
run time error.
• Exceptions are designed for run time error handling rather
than compile time error handling. Errors that occur during
compilation phase are detected by the PL/SQL compiler and
reported back to the user.
What is an Exception?
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Exception Handling

There are two types of PL/SQL exceptions in Oracle
database.
1. PreDefined / System-defined
Exceptions
2. User-Defined Exceptions
Types of Exceptions
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• System-defined exceptions are defined and maintained
implicitly by the Oracle server.These exceptions are mainly
defined in the Oracle STANDARD package.Whenever an
exception occurs inside the program.The Oracle server
matches and identifies the appropriate exception from the
available set of exceptions.
• System defined exceptions majorly have a negative error code
and error message.These errors have a short name which is
used with the exception handler.
1. System-Defined Exceptions
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Unlike System-Define Exception, User-Define Exceptions are
raised explicitly in the body of the PL/SQL block i.e more
specifically inside the BEGIN-END section)using the RAISE
Statement.
2. User-Define Exceptions
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• If our code does not have an exception handling, then each
time we execute a statement, we must verify errors in
execution
.
• If we avoid exception handling in our code, the actual errors
get missed which gives rise to some other errors.
• The exception handling allows skipping multiple verifications
in the code.
• It provides better readability of the code by isolating the error
handlers in the code
Advantages Of Exception Handling
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1. Pre-Defined Exception
• system defined exceptions are thrown by
default.
• Some of the popular System defined exceptions
are out of memory and division by
zero, having names like STORAGE_ERROR and
ZERO_DIVIDE respectively.
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Exception
Oracle
Error
SQL
CODE
Description
ACCESS_INTO_NULL ORA - 06530 -6530 This exception is raised if a null object is naturally assigned a
value.
CASE_NOT_FOUND ORA - 06592 -6592 This exception is raised if none of the options in theWHEN
clause is chosen and there is no existence of an ELSE clause.
COLLECTION_IS_NULL ORA - 06531 -6531 This exception is raised when the code tries to apply collection
methods except EXISTS to a nested table or varray which is not
initialized. It can also be raised if our code tries to assign values
to a nested table or varray which is not initialized.
DUP_VAL_ON_INDEX ORA - 00001 -1 This exception is raised if duplicate values are tried to be stored
in a column that is constrained by a unique index.
CURSOR_ALREADY_OP
EN
ORA - 06511 -6511 This exception is raised if our code tries to open an already open
cursor.
INVALID_CURSOR ORA - 01001 -1001 This exception is raised if we try to do some operations on
cursors which are not permitted.For example, attempting to
close an already closed cursor.
INVALID_NUMBER ORA - 01722 -1722 This exception is raised if the conversion to a character string to
a number does not pass as the string is representing an invalid
number.
Pre-Defined Exception
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Exception
Oracle
Error
SQL
CODE
Description
SYS_INVALID_ROWID ORA-01410 -1410 This exception is raised if the
conversion to a character string to a
universal row id does not pass as the
character string is representing an
invalid row id.
TIMEOUT_ON_RESOUR
CE
ORA-00051 -51 This exception is raised if Oracle is
waiting for a resource.
VALUE_ERROR ORA-06502 -6502 This exception is raised if a
mathematical, conversion, truncation
error happens in our program.
ZERO_DIVIDE ORA-01476 -1476 This exception is raised if we try to
divide a number by 0.
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1. Pre-Defined Exception
Exceptions Defined by Oracle i.e System Defined
Exceptions-
1 . Zero_divide
2 . value_error
3 . Invalid_Number
4 . N0_Data_Found
5 . Too_Many_Rows
6 . Dup_Val_On_Index
7 . Cursor_Already_Open
8 . Invalid_Cursor
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This Exception occurs if any number is divided by
Zero .
Eg-
a := &a; # 16
b := &b; # 0
c := a/b; # Occurs Divide_Zero
Exception
1. Zero_divide
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2. Value_error
If Value Size exceeds or if data type is mismatched then
this exception occurs
A Number(3):=50000; # Exceeding 4 digit so
Value_Error Occurs
B Number(4):=‘Swapnali’ # DataType Missmatch
Value_Error Occurs
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3. Invalid_Number
This exception occurs if invalid numeric arithmetic
operation is performed
Eg-
‘Swapnali’+10 # Invalid Arithmetic
Operation
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4. No_Data_Found
This Exception occurs if data is not found inTable
N := &roll_no;
Select stud_name # No_Data_Found Exception
From Student
Where roll_no=N;
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5. Too_Many_Rows
This exception occurs if select statement try to fetch
more than 1 record.
Eno := &deptno;
Select ename into EmpName
From employee
Where deptno=Eno;
If 10 records are there for select statement then select statement
fetches all 10 names & are assigned to EmpName but variable
EmpName takes only one at a time so too_many_rows exception
occurs
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6. Dup_Val_On_Index
This exception Occurs if we try to insert duplicate
value in primary key column.
Eg-
Create table MyFriends (id number primary key , name varchar(30));
Insert into MyFriends id values(101);
Insert into MyFriends id values(101);
# 10 already exist in table so Dup_Val_On_Index exception occurs
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7. Cursor_Already_Open
If you try to open cursor which is already open
without closing it then Cursor_Already_Open
exception occurs.
Open c1;
Open c1;
C1 is already open thus Cursor_Already_Open exception occurs.
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8. Invalid_Cursor
If you try to open cursor which is no declared then
Invalid_Cursor exception occurs.
Open C1;
Cursor C1 is not declared & you try to open itThus
Invalid_Cursor exception occurs.
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• User-defined exceptions are declared in a package, subprogram,
or within the declaration section of the PL/SQL block of code
and should be assigned names.
• Once an exception occurs, the natural flow of execution is
halted, and then the execution points to the exception section
of the PL/SQL code.
• User-defined ones have to be thrown explicitly by the RAISE
keyword.
• Thus the exception handling helps to deal with the errors that
are encountered during the run time execution and not while
compiling the program.
User-Defined Exception
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1.Declare a variable of exception data type –
This variable is going to take the entire burden on its shoulders.
2.Raise the Exception –
This is the part where you tell the compiler about the condition
which will trigger the exception.
3.Handle the exception –
This is the last section where you specify what will happen
when the error which you raised will trigger
Steps for Exception Handling
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The developers can build their own exceptions and use them for
handling errors. They can be created in the declaration part of a
subprogram and can be accessed only inside that subprogram.
An exception that is created at the package level can be used
whenever the package is accessed. A user-defined exception can be
raised by using the RAISE keyword.
Syntax -
Declare
Exception_name Exception;
Here, the exception_n is the name of the exception that we are
raising. Thus we can declare an exception by giving a name followed
by the EXCEPTION keyword. An exception can be declared in a
similar manner like variables are declared. However, an exception is
an unexpected condition and not a data item.
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Syntax for Exception Handling
DECLARE
<< declaration section >>
BEGIN
<<Block of executable code>>
IF conditionTHEN
RAISE exception_n;
END IF;
EXCEPTION
WHEN excp1THEN
<< excp1 handling block >>
WHEN excp2 THEN
<< excp2 handling block >>
........
WHEN othersTHEN
<< excp2 handling block>>
END;
The default exception is carried out with WHEN othersTHEN.
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1. Raise Statement
2. Raise_Application_Error Statement
When predefined exceptions don’t satisfy our requirement then
we define our own exception
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1.Using Raise Statement-
Exception is raised using name.This statement is
used if you want to raise exception & also want
to handle it.
2. Using Raise_Application_Error
Statement-
Exception is raised using code.This statement is
used if you want to raise exception but don’t
want to handle it.
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1.To find sum of two
number:
set serveroutput on;
declare
a int;
b int;
c int;
begin
a:=&a;
b:=&b;
c:=a+b;
dbms_output.put_line('sum
of a and b'||c);
end;
2.To find greatest number among
two:
set serveroutput on
declare
a int;
b int;
c int;
begin
a:=&a;
b:=&b;
if(a>b)
then
dbms_output.put_line('a is greater');
else
dbms_output.put_line('b is greater');
end if;
end;

Declare
a number(2);
b number(2);
c number(4);
one_divide exception;
Begin
a:= &a;
b:= &b;
if b=1 then
raise one_divide;
end if ;
c:= a/b;
dbms_Output.put_line(c);
Exception
when zero_divide then
dbms_output.put_line('zero_Divide');
when one_divide then
dbms_output.put_line('one_divide');
when others then
dbms_output.put_line(sqlerrm);
End; /
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2. Using Raise_Application_Error Statement-
Declare
vage number(10);
age number:=&vage;
Begin
if age < 18 then
Raise_Application_Error(-20008,'Age must be greater than 18 !');
end if;
dbms_output.put_line(‘Voting Allowed after completing 18 years');
Exception
when others then
dbms_output.put_line(sqlerrm);
End;
/
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Example.
CREATETABLE CITIZEN (
ID INT NOT NULL,
NAMEVARCHAR (15) NOT NULL,
AGE INT NOT NULL,
PRIMARY KEY (ID)
);
We have created the CITIZEN table with the help of the SQL statement given below.
SELECT * FROM CITIZEN
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Insert values to this table with SQL statements given below:
INSERT INTO CITIZENVALUES (1,‘Swapnali', 26);
INSERT INTO CITIZENVALUES (8,‘Rugveda', 15);
INSERT INTO CITIZENVALUES (5,‘Shree', 37);

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Coding ImplementationWith Exception Handling:
DECLARE
citizen_id citizen.id%type;
citizen_name citizen.name%type;
citizen_age citizen.age%type := 9;
BEGIN
SELECT id, name INTO citizen_id, citizen_name
FROM citizen
WHERE age = citizen_age;
DBMS_OUTPUT.PUT_LINE ('Citizen id is: '|| citizen_id);
DBMS_OUTPUT.PUT_LINE ('Citizen name is: '|| citizen_name);
EXCEPTION
WHEN no_data_foundTHEN
dbms_output.put_line ('No citizen detail found');
WHEN othersTHEN
dbms_output.put_line ('Errors');
END;
/

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Student Activity
1. Using Raise statement create user defined exception which raise
an error if sum of 2 numbers is greater than 5000.
2. Using Raise statement create user defined exception which raise
an exception if age <18 and displays ‘Your are not Allowed for
Voting This Year ! ’ Message.
3. Using Raise_Application_Error statement create your own
exception code for “Salary must be above 50000 for Loan
Approval !” message.
4. Create table with your name .Add columns Roll_no Name Marks
.Enter some records in table. Execute various queries on that table
& if records are not available raise no_data_found exception.

Exception Handling

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Exception Handling