SQL & PL/SQL :: Function Returning Top Value If Not Exists Then Next One?

how to write a function that returns top value if not exists then next top for combination of customer_id and hierarchy.For instance :

If I've got table

customer_id ,hierarchy, function_code
123 |1 | Z1
123 |2 |67
123 |3 |5B
678 |10 |S2
678 |11 |Z2
345 |2 |11

For the customer ID 123 I want to return Z1, for customer 678 I want to return S2 and for customer ID 345 I want 11

Problem is that I'm new to the concept of looping. I know how to write a function that accepts customer_id as a value write a cursor and then check IF hierarchy = 1 the return FUNCTION_CODE IF hierarchy - 2 THEN ...

but I need something more universal as some of the customers may have hierarchy function 1 and that would be the top one for him but others might have function of hierarchy 10 as top and checking all of the possibilities using if would be just stupid. So how to write something universal ? And of course if function did not find any customer_id then return null.