Exception

         Error handling: action to be peformed when error occours.
Pre-defined exception
.....................
Pre-defined exception: Zero_divide
example
Without Excepion
...
begin
dbms_output.put_line('print 1');
dbms_output.put_line(3/0);
dbms_output.put_line('print 3');
end;
/
Error:
ORA-01476: divisor is equal to zero
...
With Excepion
...
begin
dbms_output.put_line('print 1');
dbms_output.put_line(3/0);
dbms_output.put_line('print 3');
exception
when zero_divide then
dbms_output.put_line('print 2');
end;
/
Output:
print 1
print 2
Statement processed.

Pre-defined exception: Dup_val_on_index

In table t1(a)
a is primary key.
primary key does not accept duplicate values and null values.
when we insert the duplicate values 
...
example
Without Excepion
...
begin
insert into t1 values(1);
insert into t1 values(2);
insert into t1 values(1);
end;
/
Error:
ORA-00001: unique constraint (HR.Q1) violated
...
With Excepion
...
begin
insert into t1 values(1);
insert into t1 values(2);
insert into t1 values(1);
exception
when dup_val_on_index then
dbms_output.put_line('print 3');
end;
/
output:
print 3
Statement processed.
...

Pre-defined exception: Too_many_rows
Example
Without Excepion
...
declare
i employees.first_name%type;
begin
select first_name into i from employees where rownum<=5;
dbms_output.put_line(i);
end;
/
Error:
ORA-01422: exact fetch returns more than requested number of rows
...
With Excepion
...
declare
i employees.first_name%type;
begin
select first_name into i from employees where rownum<=5;
dbms_output.put_line(i);
exception
when too_many_rows then
dbms_output.put_line('print 5');
end;
/
Output:
print 5
Statement processed.
...
Pre-defined exception: No_data_found
Example
Without Excepion
...
declare
i employees.first_name%type;
begin
select first_name into i from employees where rownum<=0;
dbms_output.put_line(i);
end;
/
Error:
ORA-01403: no data found
...
With Excepion
...
declare
i employees.first_name%type;
begin
select first_name into i from employees where rownum<=0;
dbms_output.put_line(i);
exception
when no_data_found then
dbms_output.put_line('print 6');
end;
/
Output:
print 6
Statement processed.
...

Pre-defined exception: Others

     Others handler must be in the last among the exception handlers of a block.
Example:
...
declare
i employees.first_name%type;
begin
select first_name into i from employees where rownum<=0;
dbms_output.put_line(i);
exception
when too_many_rows then
dbms_output.put_line('print 5');
when others then
dbms_output.put_line('print 6');
end;
/
Output:
print 6
Statement processed.
...

SqlCode
........
Example
...
begin
dbms_output.put_line('print 1');
dbms_output.put_line(3/0);
exception
when others then
dbms_output.put_line(sqlcode);
end;
/
...
OutPut;
print 1
-1476..............sqlcode
Statement processed.
...
Sqlerrm- Error Name
........
Example:
...
begin
dbms_output.put_line('print 1');
dbms_output.put_line(3/0);
exception
when others then
dbms_output.put_line(sqlerrm);
end;
/
Output:
...
print 1
ORA-01476: divisor is equal to zero.............Error Name

Statement processed.
...
Dbms_utility.format_error_stack   -Error Name
...............................
Example:
...
begin
dbms_output.put_line('print 1');
dbms_output.put_line(3/0);
exception
when others then
dbms_output.put_line(dbms_utility.format_error_stack);
end;
/
Output:
...
print 1
ORA-01476: divisor is equal to zero.............Error Name

Statement processed.

Dbms_utility.format_error_backtrace - Error line 
...................................
Example:
...
begin
dbms_output.put_line('print 1');
dbms_output.put_line(3/0);
exception
when others then
dbms_output.put_line(Dbms_utility.format_error_backtrace);
end;
/
...
Output:
...
print 1
ORA-06512: at line 3

Statement processed.

User Defined Exception
......................
To throw the Error- Use 'raise'

Example:
...
declare
greens exception;
begin
dbms_output.put_line('print 1');
raise greens;
dbms_output.put_line('print 2');
exception
when greens then
dbms_output.put_line('print 3');
end;
/
...
Output:
print 1
print 3
Statement processed.
...
Non-predefined Exception-  pragma exception_init
........................
Example
...
declare
greens exception;
pragma exception_init(greens,-01476);
begin
dbms_output.put_line('print 1');
dbms_output.put_line(6/0);
dbms_output.put_line('print 2');
exception
when greens then
dbms_output.put_line('print 3');
end;
/ 
Output:
print 1
print 3
Statement processed.
...
User Defined Error
..................
User can create the error using 'raise_application_error'
Error code range can be
-20000 to -20999
Example:
...

begin
dbms_output.put_line('print 1');
raise_application_error(-20000,'User Error');
dbms_output.put_line('print 2');
end;
/
Output:
...
ORA-20000: User Error.