create table imp_t1
(col1 integer,
col2 varchar2(10),
col3 date default sysdate);

create table imp_t2
col4 integer,
col5 varchar2(10));

insert into imp_t1 values(1,'Naresh',null);
insert into imp_t1 values(2,'Naresh',null);
insert into imp_t1 values(3,'Naresh',null);

insert into imp_t2 values(1,'Raja');
insert into imp_t2 values(2,'Raja');
insert into imp_t2 values(3,'Raja');

DECLARE
  v_flag   number := :b1;
  v_count  number;
BEGIN
  IF v_flag = 1 then
    SELECT count(*) into v_count from table_1;
  ELSE
    SELECT count(*) into v_count from table_2;
  END IF;
END;
/

SELECT * from IMP_T1 where &flag = 1;
SELECT * FROM IMP_T2 where &flag != 1;

select *
from (
		select *
		from t1
		where &flag = 1
	) cross join (
		select *
		from t2
		where not &flag = 1
	)