IF EXISTS condition not working with PLSQL

IF EXISTS() is semantically incorrect. EXISTS condition can be used only inside a SQL statement. So you might rewrite your pl/sql block as follows:

declare  l_exst number(1);begin  select case            when exists(select ce.s_regno                          from courseoffering co                         join co_enrolment ce                           on ce.co_id = co.co_id                        where ce.s_regno=403                           and ce.coe_completionstatus = 'C'                           and ce.c_id = 803                          and rownum = 1                        )           then 1           else 0         end  into l_exst  from dual;  if l_exst = 1   then    DBMS_OUTPUT.put_line('YES YOU CAN');  else    DBMS_OUTPUT.put_line('YOU CANNOT');   end if;end;

Or you can simply use count function do determine the number of rows returned by the query, and rownum=1 predicate - you only need to know if a record exists:

declare  l_exst number;begin   select count(*)      into l_exst     from courseoffering co          join co_enrolment ce            on ce.co_id = co.co_id    where ce.s_regno=403       and ce.coe_completionstatus = 'C'       and ce.c_id = 803      and rownum = 1;  if l_exst = 0  then    DBMS_OUTPUT.put_line('YOU CANNOT');  else    DBMS_OUTPUT.put_line('YES YOU CAN');  end if;end;

Unfortunately PL/SQL doesn't have IF EXISTS operator like SQL Server. But you can do something like this:

begin  for x in ( select count(*) cnt               from dual               where exists (                select 1 from courseoffering co                  join co_enrolment ce on ce.co_id = co.co_id                 where ce.s_regno = 403                    and ce.coe_completionstatus = 'C'                    and co.c_id = 803 ) )  loop        if ( x.cnt = 1 )         then           dbms_output.put_line('exists');        else            dbms_output.put_line('does not exist');        end if;  end loop;end;/