Getting a query result taken from the same data but with temporary var - sql

Question

I got a simple thing to do.
Well, maybe not, but someone somewhere surely can help me out : P
I got a simple data structure that contains
expedition date
delivery date
transaction type
I would need to create a query which could
order the rows by a date specific to the transaction type.
(ie : using the expedition date for transaction of type "selling", and delivery date for transaction of type "purchasing")
I was wondering if there was a more efficient way to do this than
by fetching 2 times the same data with different clause where(while adding a column used to order them(tempDate)) and then using another select to encompass these 2 queries to which I would add the order clause on the tempDate.
--> the initial fetching I would do 2 times works on many tables(many, many, many joins)
Basically my current solution is :
Select * from
(
Select ...
date_exp as dateTemp;
from ...
where conditions* And dateRelatedCondition
UNION
Select ...
date_livraison as dateTemp;
from ...
Where conditions* And NOT(dateRelatedCondition)
) as comboSelect
Order By MIN(comboSelect.dateTemp)
OVER(PARTITION BY(REF_product)),
(REF_product),
comboSelect.dateTemp asc;
*
->Those conditions are the same in both inner Select query
Thank you for your time.

Answer 1

Without the UNION:
dateRelatedCondition should be removed from WHERE and put to the SELECT like:
CASE WHEN dateRelatedCondition THEN date_exp ELSE date_livraison END as dateTemp
Without the subquery:
in ORDER BY you need the same expression in the window function:
Order By MIN(CASE WHEN dateRelatedCondition THEN date_exp ELSE date_livraison END)
OVER(PARTITION BY(REF_product)),
(REF_product),
dateTemp asc

Answer 2

You mean like this?:
ORDER BY CASE
WHEN TransactionType = 'Selling' THEN ExpeditionDate
WHEN TransactionType = 'purchasing' THEN DeliveryDate
END