Instead of using UNIONs, you might also be able to do this by using multiple joins to the LOV_Details table using aliases. It would look something like this:
SELECT p.employee_number,
p.role_comp_emp_final_rating,
Lemp.ov_text_en empText,
p.role_comp_emp_final_rating
Lrating.ov_text_en ratingText
FROM pa_master_details P
Inner Join lov_details Lemp
ON p.role_comp_emp_final_rating = Lemp.lov_value
inner join lovdetails Lrating
on p.role_comp_lm_final_rating = Lrating.lov.value
-Dell