using case inside the ON clause of a join - sql

Question

I have a question: what is the right way to use a CASE statement in the ON clause of a JOIN?
(I saw a couple of similar questions had been asked by others, but I don't see how to replicate the solutions in my scenario.)
I try to connect 2 instances of the same table (self join).
The table contains a balance value of each customer for each day (row per customer per day, each customer appears multiple times- since the first day of his activity and till he/she closes the account)
For each customer i need to find what is the balance today and in another column - what was the balance on the "base date". The default for the base date is the 31 -dec-2019 BUT if the customer comes from a specific branch (BRANCH "X") then the base date should be 31-mar-2020 (instead of 31-dec-2019).
So what I'm trying to do is to write something like this:
select
B.branch_name,
B.customer_id,
B.balance as current_balance,
B1.balance as base_date_balance,
from
balance B inner join balance B1
on B.customer_id=B1.customer_id
and B.date = '20apr2020'
and B1.date= (case when B.branch_name = 'X' then '31-mar-2020' else '31-dec-2019' end)
I get that this is not the right way to do this, but I can't figure it out what the right way is.
Thanks in advance for all the answers and the help, highly appreciate ! :)

Answer 1

Use window functions!:
select b.*,
max(case when branch = 'B' and date = date '2019-03-31' then balance
when date = date '2019-12-31' then balance
end) over (partition by customer_id, branch_name
) as base_date_balance
from balance b;
This should have better performance than a JOIN with an OR or CASE condition.
Alternatively, the join would look like:
from b join
b bbase
on bbase.customer_id = b.customer_id and
bbase.branch_name = b.branch_name and
( (bbase.branch_name = 'B' and bbase.date = date '2019-03-31') or
(bbase.branch_name <> 'B' and bbase.date = date '2019-12-31')
)

Answer 2

Select b.* from table
where rownum=1
and date<(select date from table where cust=... order by date desc)
order by date desc
This will give you the previous transaction according to the criteria. The subquery returns the latest transaction.
The next step will depend on the database. On Oracle you use:
Select (select latest_transactions)-(select previous_tranaction) from dual.
If you need more info let me know. I am writing from my phone.