Value for two days - sql

Question

I have a table that shows the interest rate on an account on a given day. I'm looking to query the interest rate on one day and the interest rate for the next day. Specifically, I'm looking to find accounts where the interest rate changed from one day to the next.
My data looks like this
Account Number / Loan Number / Date / Interest Rate
1234 / 5656 / 1/1/18 / 12%
1234 / 5656 / 1/2/18 / 12%
1234 / 5656 / 1/3/18 / 0%
1234 / 5656 / 1/4/18 / 0%
I want the query to return just the two days where the interest rate changed from 12% to 0%.
With the above data, it would return the following:
Account Number / Loan Number / Date / Interest Rate / Next Day / Next Day Rate
1234 / 5656 / 1/2/18 / 12% / 1/3/18 / 0%
The code I'm using (see below) is returning the same Date multiple times and modifying the "Next Day Rate" by some factor I cannot identify.
This is what I have so far.
select
tc.loanaccountid AS 'Account'
, l.LoanNumber AS 'Loan Number'
, tc.trialbalancedate AS 'Date'
, tc.interestrate AS 'Interest Rate'
, tb.trialbalancedate AS 'Next Day'
, tb.interestrate AS 'Next Day Rate'
from dbo.dailytrialbalance tc
join dbo.loanaccount l on tc.loanaccountid = l.loanaccountid
left join dbo.dailytrialbalance tb on dateadd(day, 1, tc.trialbalancedate) =
tb.trialbalancedate
where tc.PortfolioCodeId = '10'
and tc.interestrate = '0'
and tb.interestrate > '0'
I'm still learning SQL, so any help is appreciated. Thanks.

Answer 1

Here is a way with LEAD for SQL Server 2012 onward. I added the (SELECT NULL) as a place holder for something more realistic to determine the difference since you have duplicate data for 1/2/18:
with cte as(
select
tc.loanaccountid AS 'Account'
, l.LoanNumber AS 'Loan Number'
, tc.trialbalancedate AS 'Date'
, tc.interestrate AS 'Interest Rate'
, NextDay = lead(tc.trialbalancedate) over (partition by tc.loanaccountid order by tc.trialbalancedate, (select null))
, NextDayRate = lead(tc.interestrate) over (partition by tc.loanaccountid order by tc.trialbalancedate, (select null))
from dbo.dailytrialbalance tc
join dbo.loanaccount l on tc.loanaccountid = l.loanaccountid)
select *
from cte
where NextDayRate = 0 and [Interest Rate] != 0
Here is it in action, with your test data:
declare #testData table (AccountNumber int, LoanNumber int, Date date, InterestRate varchar(3))
insert into #testData
values
(1234,5656,'20180101','12%'),
(1234,5656,'20180102','12%'),
(1234,5656,'20180103','0%'),
(1234,5656,'20180104','0%')
;with cte as(
select
AccountNumber
,LoanNumber
,[Date]
,InterestRate
,NextDay = lead([Date]) over (partition by AccountNumber order by [Date])
,NextDayRate = lead(InterestRate) over (partition by AccountNumber order by [Date])
from #testData)
select *
from cte
where NextDayRate = '0%' and InterestRate != '0%'