How to select only one row that satisfies one of multiple ordered conditions - sql

Question

I have customers that have multiple addresses. Each customer/address combination has its own line in the database (Oracle) table.
I am trying to achieve a query in which, if the customer has a 'Main Address', I display only the Main Address,
otherwise if he has a Shipment Address, I display only the Shipment Address,
otherwise if he has a 'Secondary Address', I display the Secondary Address,
otherwise I display nothing.
This order is important, and the problem is that the entries in the database are in no specific order, meaning that the same customer might be found to have a Shipment Address first, and a Main Address later on. Therefore I don't simply need the first row that satisfies one of the conditions...
I tried this, but it returns a result for each line, e.g. multiple results for one person:
CASE WHEN ADR = 'MAIN' THEN 'MAIN'
WHEN ADR = 'SHIPMENT' THEN 'SHIPMENT'
WHEN ADR = 'SECONDARY' THEN 'SECONDARY'
ELSE null
END AS Adressart
To clarify, the input looks as follows:
CUSTOMER_NR ADDRESS_TYPE
1 SHIPMENT
1 MAIN
2 SHIPMENT
3 SECONDARY
3 SHIPMENT
4 SECONDARY
The results would look like this:
CUSTOMER_NR ADDRESS_TYPE
1 MAIN
2 SHIPMENT
3 SHIPMENT
4 SECONDARY

Answer 1

I think you could use multiple joins to the address table and by using coalesce return only the one address from the order
The query would look something like:
SELECT customer_nr, coalesce(a1.address, a2.address, a3,address) AS address
FROM customer AS c
LEFT JOIN address AS a1 where a1.customer_nr = c.customer_nr and a1.address_type = 'MAIN'
LEFT JOIN address AS a2 where a2.customer_nr = c.customer_nr and a2.address_type = 'SHIPMENT'
LEFT JOIN address AS a3 where a3.customer_nr = c.customer_nr and a3.address_type = 'SECONDARY'

Answer 2

The following query would get you the desired result:
SELECT
CUSTOMER_NR,
ADDRESS_TYPE
FROM
(
select
customer_nr,
address_type,
row_number () over (partition by customer_nr order by
case address_type
when 'MAIN' then 1
when 'SHIPMENT' then 2
when 'SECONDARY' THEN 3
else 4
end) rn
from addresses
)
WHERE rn = 1;
First, the addresses are sorted using the priority with a CASE statement. Then, only the address types that have rn = 1 (the address type of highest priority) are selected.
SQL Fiddle demo