how to use insert statement with where clause from diff table in oracle without using plsql

Question

I want to insert like below

If 
   select *from class where student_name='A' ;
then
   Insert into student (Studnetname varchar2,rollno integer) values('A',1);

without using PL/SQL

Answer 1

Wouldn't that be as simple as

insert into student (studentname, rollno)
select student_name, rollno
from class
where student_name = 'A'
  and rollno = 1;

If such a student (whose values satisfy the WHERE clause) exists, a row will be inserted. Otherwise, nothing will happen.

Answer 2

I want direct insert statement without select

The closest you will get to meeting this requirement is something like this:

Insert into student (Studnetname, rollno) 
select 'A',1
from dual 
where exists (select null from class where student_name = 'A' )
;

This will insert one row into STUDENT if there is at least one record in CLASS matching the given criterion.