MSSQL Subtract two rows from each other in union - sql

Question

So I have a successful query that shows the first row of last month, and the first row of this month. This query shows fields that have ongoing counters for electricity usage.
I need the difference of a particular field, and then multiply that by .54802 and round to nearest hundredth.
SELECT *
FROM
(
SELECT TOP (1) *
FROM Datebase
WHERE DATEPART(M, Timestamp) = DATEPART(M, DATEADD(M, -1, GETDATE()))
AND DATEPART(Yyyy, Timestamp) = DATEPART(Yyyy, DATEADD(M, 0, GETDATE()))
ORDER BY Timestamp ASC
) AS A
UNION
SELECT *
FROM
(
SELECT TOP (1) *
FROM Datebase
WHERE DATEPART (M, Timestamp) = DATEPART(M, DATEADD(M, 0, GETDATE()))
AND DATEPART(Yyyy, Timestamp) = DATEPART(YYYY, DATEADD(M, 0, GETDATE()))
ORDER BY Timestamp ASC
) AS B;
This outputs:
| ID | Timestamp | 7000AV119 |
---------------------------------------------------
| 1 | 2018-08-01 00:00:03.000 | 3675.59 |
| 2 | 2018-09-01 00:00:03.000 | 3750.93 |
I need it to show:
| ID | Timestamp | 7000AV119 | Difference
-----------------------------------------------------------
| 1 | 2018-08-01 00:00:03.000 | 3675.59 | 0
| 2 | 2018-09-01 00:00:03.000 | 3750.93 | 41.29

Answer 1

I would get the two records as:
select t.*
from (select t.*,
row_number() over (partition by year(timestamp), month(timestamp) order by timestamp asc) as seqnum
from Datebase t
where timestamp >= dateadd(day, 1, eomonth(getdate(), -2))
) t
where seqnum = 1;
Then I would use lag():
select t.*,
round(([7000AV119] - lag([7000AV119]) over (order by timestamp)
) * 0.54802,
2
)
from (select t.*,
row_number() over (partition by year(timestamp), month(timestamp) order by timestamp asc) as seqnum
from Datebase t
where timestamp >= dateadd(day, 1, eomonth(getdate(), -2))
) t
where seqnum = 1;
This returns NULL rather than 0 for the first row. That makes more sense to me.

Answer 2

Try with lag function
with testcte as
(
select *
from (select top(1) * from DATABASE
WHERE DATEPART(m, Timestamp) = DATEPART(m, DATEADD(m, -1, getdate()))
AND DATEPART(yyyy, Timestamp) = DATEPART(yyyy, DATEADD(m, 0, getdate()))
order by Timestamp ASC) a
union
select *
from (select top(1) * from DATABASE
WHERE DATEPART(m, Timestamp) = DATEPART(m, DATEADD(m, 0, getdate()))
AND DATEPART(yyyy, Timestamp) = DATEPART(yyyy, DATEADD(m, 0, getdate()))
order by Timestamp ASC) b
)
select id,Timestamp,7000AV119,7000AV119- LAG(7000AV119,1,0) OVER (ORDER BY id) AS Diff
from testcte