Resolved: I am not sure what minus means, may you illustrate

Try running the query with and without the minus sign, then compare the result sets. it should be pretty clear then what it is doing, though i'm not sure why the chose to demonstrate that here...

Thanks Nicholas Burmeister

Also ordering by -a.emp_no desc is better than ordering by a.emp_no asc as the first one does not display null values first but the second one does