create table table1
as
select level lvl
  from dual connect by level<1000000;
  
create table table2
as
select cast(null as number) lvl from dual where 1=2;

-- вариант 1
with t2 as (select count(*) cnt from table2)
select t1.*
  from table1 t1, t2
 where t2.cnt = 0
    or (t2.cnt > 0 and t1.lvl in (select lvl from table2 t3));
    
-- вариант 2    
with t2 as (select null lvl 
             from dual
            where 0 = (select count(*) cnt from table2)
            union all
            select lvl from table2)
select t1.*
  from table1 t1, t2
 where t1.lvl = nvl(t2.lvl, t1.lvl);

with t2 as (select lvl 
              from table1
             where 0 = (select count(*) cnt from table2)
             union all
            select lvl from table2)
select t1.*
  from t2, table1 t1
 where t2.lvl = t1.lvl(+);