Calculate days from the last time a customer has zero balance - sql

Question

I have a Table with these columns :
TransID, CustomerID, Date, Credit, Debit, CurrentBalance
I want to know how many days have passed since the customer had a clear balance, because I don't give credit if they have not cleared their balance in the last 14 days,
let's speak about a specific customer:
TransID, CustomerID, Date, Credit, Debit, CurrentBalance
1 1 01/01/2014 0 50 50
2 1 01/05/2014 50 0 0
3 1 06/28/2014 0 100 100
Now on 6/29/14 they have only 1 day since their balance was clear, but if I calculate from the last row with CurrentBalance = 0, it is more than 175 days

Answer 1

Logically, the last time the balance was zero was the instant before the last sale was made when the balance was zero, which can be identified by the sale amount equalling the balance after the sale, ie Debit = CurrentBalance - this confition can only happen when the balance before the sale was zero.
select
c.id customerid,
coalesce(datediff(day, max(t.date), getdate()), 0) days_clear
from customer c
left join transaction t on t.CustomerID = c.id
and Debit = CurrentBalance
group by customerid
Using the customer table and a left join to the transaction table allows for the case when customer has never made a transaction (so his day count is zero).

Answer 2

Is this what you are looking for?
select customerid,
datediff(day, max(case when balance = 0 then date end), getdate())
from table t
group by customerid;
This returns the number of days since the most recent 0 balance record.
EDIT:
Now I think I understand. The problem is that the 0 balance lasts until 6/18/2014. With SQL Server 2012 or later, we can handle this with lead():
select customerid,
datediff(day, max(case when balance = 0 then nextdate end), getdate())
from (select t.*, lead(date) over (partition by customerid order by date) as nextdate
from table t
) t
group by customerid;
T