declare
  table_or_view_doesnt_exist exception;

  pragma exception_init ( table_or_view_doesnt_exist, -942 );
begin
  begin
    execute immediate 'drop table employees_0';
  exception when table_or_view_doesnt_exist then null; end;

  begin
    execute immediate 'drop table employees_2';
  exception when table_or_view_doesnt_exist then null; end;
end;
/
create table employees_0 as select * from hr.employees

/
alter table employees_0
  add constraint employees_0_pk primary key (employee_id)
  using index
/
create table employees_2 as select * from hr.employees where 1=2
/

alter table employees_2
  add constraint employees_2_pk primary key (employee_id)
  using index
/