SQL查询返回两个最近日期的记录之间的差额

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:

1. 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.

2. 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