SQL & PL/SQL :: Query For Eliminating Union

I have one scenario in which i want to write the sql,but not able to write correct qry,

tables
1-emp
2-emp_hist
3-dept
4-dept_hist.

i want to retrieve data from the emp and hist able based on some conditions,but if data is not present in emp and dept tables.then fetch data from emp_hist and dept_hist.I have written below qry which is working fine,but my prob is i want to provide my emp_id only one place.
else i have to change my java code.

select * from emp,dept where emp.emp_id=dept.dept_id and emp_id=5
union
select * from emp_hist,dept_hist where emp_hist.emp_id=dept_hist.dept_id and emp_id=5