Join multiple tables Oracle - sql

Question

I have 3 tables, a purchase (p), a refund (r) and a send to finance software batching table (b).
Set up like this:
purchases
=========
order_number VARHCAR(10)
order_id (number)
order_date (date)
refunds
=======
refund_id (number)
order_id <-- relates to purchases table (number)
refund_date (date)
send_to_finance_batch
=====================
generic_id (number) <-- this can be filled in by an number of tables
type (text)
status_reason (text)
satus_id (number)
(more or less)
What I need to figure out is how to do a left join (since the generic_id can come from several entities) refunds to purchases?
Right now I have this:
SELECT p.order_number,
b.generic_id,
b.status_id,
b.status_reason,
FROM send_to_finance_batching b
LEFT JOIN purchases p ON b.generic_id = p.order_id
WHERE...
refund_id OR order_id will equal the generic_id so I need to relate generic_id to refund_id and then from the refunds table order_id to orders table.
My desired output is to be able to fill in p.order_number, right now I have the other columns but since order_number only exists on the purchase table id like to link it back to the refunds table by joining on the batching table.
What I have right now is this:
Order number | type | generic_id | status_reason | status_id
==============================================================
| refund | 1234567890 | 'stuck' | 99
WH763HWW3499 | purch | 7534567654 | 'error' | 99
I would like to be able to fill in the order number on the refunds, but since generic_id relates to the refund_id I need to be able to use the order_id from refunds to get the info from purchases and have a complete query

Answer 1

Option 1: Using UNION
In this case you would lose rows that neither can be joined to purchases nor refunds.
SELECT p.order_number,
b.generic_id,
b.status_id,
b.status_reason
FROM send_to_finance_batching b
JOIN purchases p ON b.generic_id = p.order_id
WHERE...
UNION
SELECT p.order_number,
b.generic_id,
b.status_id,
b.status_reason
FROM send_to_finance_batching b
JOIN refunds r ON b.generic_id = r.refund_id
JOIN purchases p ON r.order_id = p.order_id
WHERE...
Option 2: JOIN purchases multiple times and using COALESCE
SELECT coalesce(p.order_number, p2.order_number) order_number,
b.generic_id,
b.status_id,
b.status_reason
FROM send_to_finance_batching b
LEFT JOIN purchases p ON b.generic_id = p.order_id
LEFT JOIN refunds r ON b.generic_id = r.refund_id
LEFT JOIN purchases p2 ON r.order_id = p2.order_id
WHERE...
Option 3: Use COALESCE on JOIN condition
SELECT p.order_number,
b.generic_id,
b.status_id,
b.status_reason
FROM send_to_finance_batching b
LEFT JOIN refunds r ON b.generic_id = r.refund_id
LEFT JOIN purchases p ON coalesce(r.order_id, b.generic_id) = p.order_id
WHERE...

Answer 2

Outer join the two tables, but consider the type column in order not to join the wrong entity. Then use COALESCE to display the order number from the referenced row.
SELECT
COALESCE(p.order_id, r.order_id) AS order_id,
fb.generic_id,
fb.status_id,
fb.status_reason
FROM send_to_finance_batching fb
LEFT JOIN purchases p ON fb.generic_id = p.order_id AND fb.type = 'purch'
LEFT JOIN refunds r ON fb.generic_id = r.refund_id AND fb.type = 'refund'
ORDER BY order_id;