Oracle 18c

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create table your_table (id number, foo varchar2(30));
declare
  cnt pls_integer;
begin
  select count(*)
  into cnt
  from dual
  where exists (select null from your_table);

  if cnt = 0 then
    dbms_output.put_line('Inserting...');
    insert into your_table (id, foo) values (1, 'bar');
  else
    dbms_output.put_line('Not inserting, data exists');
  end if;
end;
/
1 rows affected

dbms_output:
Inserting...

declare
  cnt pls_integer;
begin
  select count(*)
  into cnt
  from dual
  where exists (select null from your_table);

  if cnt = 0 then
    dbms_output.put_line('Inserting...');
    insert into your_table (id, foo) values (2, 'baz');
  else
    dbms_output.put_line('Not inserting, data exists');
  end if;
end;
/
1 rows affected

dbms_output:
Not inserting, data exists

select * from your_table;


ID
FOO


1
bar


rollback;
insert into your_table (id, foo)
select 1, 'bar'
from dual
where not exists (select null from your_table);
1 rows affected

insert into your_table (id, foo)
select 2, 'baz'
from dual
where not exists (select null from your_table);
select * from your_table;


ID
FOO


1
bar