SQL Query to return the difference between records of two most recent dates - sql

Question

I have the following table:
**TABLE1**
RecordID UserID UserName Balance TranDate
---------------------------------------------------------------
100 10001 John Doe 10213.00 2013-02-12 00:00:00.000
101 10001 John Doe 1932.00 2013-04-30 00:00:00.000
102 10001 John Doe 10213.00 2013-03-25 00:00:00.000
103 10001 John Doe 14514.00 2013-04-12 00:00:00.000
104 10001 John Doe 5430.00 2013-02-19 00:00:00.000
105 10001 John Doe 21242.00 2010-02-11 00:00:00.000
106 10001 John Doe 13342.00 2013-05-22 00:00:00.000
Now what i'm trying to do is to query the two most recent transactions and arrive at this data:
RecordID UserID UserName Balance TranDate
---------------------------------------------------------------
106 10001 John Doe 13342.00 2013-05-22 00:00:00.000
101 10001 John Doe 1932.00 2013-04-30 00:00:00.000
Then using the data above I would like to compare the balances to show the difference:
UserID UserName Difference
---------------------------------------------------------------
10001 John Doe -11410.00
This just shows the difference between the two previous balances (the latest and the balance before the latest)
Now I have the following query below. This works okay to show the two most recent transactions.
SELECT
TOP 2 *
FROM Table1
WHERE UserID = '1001'
ORDER
BY TranDate DESC
Now my issues are:
Is the sql above safe to use? I am just relying on the sorting of the TranDate by the ORDER BY DESC keyword and I am not so sure if this is very much reliable or not.
How do I select the difference between the two Balances (Row 2 - Row 1 )? I was looking for some answers online and I find stuff about self-joining. I tried it but it doesn't show me my desired output.
EDIT:
This is the closest I can get to my desired result. Can someone help me out on this please? Thanks!
DECLARE #SampleTable TABLE
(
UserID INT,
UserName VARCHAR(20),
Balance DECIMAL(9,2) DEFAULT 0
)
INSERT
INTO #SampleTable
(UserID, UserName, Balance)
SELECT
TOP 2 UserID,
UserName,
Balance
FROM Table1
WHERE UserID = '1001'
ORDER
BY TranDate DESC
SELECT A.UserID,
A.UserName,
B.Balance - A.Balance AS Difference
FROM #SampleTable A
JOIN #SampleTable B
ON A.UserID = B.UserID
Thanks a lot!

Answer 1

You should be able to use something like the following assuming SQL Server as the RDBMS:
;with cte as
(
select recordid, userid, username, balance, trandate,
row_number() over(partition by userid order by trandate desc) rn
from table1
)
select c1.userid, c1.username,
c1.balance - c2.balance diff
from cte c1
cross apply cte c2
where c1.rn = 1
and c2.rn = 2;
See SQL Fiddle with demo.
Or this could be done using an INNER JOIN on the row_number value:
;with cte as
(
select recordid, userid, username, balance, trandate,
row_number() over(partition by userid order by trandate desc) rn
from table1
)
select c1.userid, c1.username,
c1.balance - c2.balance diff
from cte c1
inner join cte c2
on c1.rn + 1 = c2.rn
where c1.rn = 1
See SQL Fiddle with Demo