Learning PL/SQL ... a query

Question

hi all,
i v jus started learning pl/sql. i v a question
the following is my pl/sql block
set serveroutput on;
declare
name varchar(50);
empid number := &emp;
begin
select last_name into
name
from hr.employees
where employee_id=empid;
dbms_output.put_line('name of employee is '||name);
exception
when no_data_found
then
dbms_output.put_line('no data for employee with the id:'||empid);
when others
then
dbms_output.put_line('error');
end;when i run this code i m asked to enter emp. if i type the emp no. which exists in the db then i get the name of the employee and if that emp_id is not present in the database i get a output saying "no data for employee with the id:<empid>".
now my question is ... when asked to enter the emp_id if i just hit enter i.e. i enter a null value, the exception block does not catch the error.
it throws an error
Enter value for emp:
old   3: empid number := &emp;
new   3: empid number := ;
empid number := ;
ERROR at line 3:
ORA-06550: line 3, column 17:
PLS-00103: Encountered the symbol ";" when expecting one of the following:
( - + case mod new not null <an identifier>
<a double-quoted delimited-identifier> <a bind variable> avg
count current exists max min prior sql stddev sum variance
execute forall merge time timestamp interval date
<a string literal with character set specification>
<a number> <a single-quoted SQL string> pipe
The symbol "null" was substituted for ";" to continue.what am i missing??

Answer 1

are we dependent on the front end so that a null value is not sent to the db??Your example used simple substitution variables directly in a sql (or pl/sql) statement. When you entered nothing and hit enter, SQL Plus basically did nothing and sent the block to the db for execution. That's just the way SQL Plus works.
Your application will not behave in this manner.
Ideally, your program (Java or whatever) will use bind variables (I hope). In that case, the statement sent to the database will not have the syntax error you saw above. (however, it will not return any results because of the null variable).