Count records where the date is last month - sql

Question

I have a table like this
And then I want to add a column besides Total column. The column is LastMonthTotal with the same record but the date is last month before. How can I do that?
Here's the DDL
CREATE TABLE [dbo].[ERA_Report2]
(
[RESULT] [nvarchar](255) NULL,
[CLASSIFICATION_2] [nvarchar](255) NULL,
[ARMADA] [nvarchar](255) NULL,
[CDAREA] [nvarchar](255) NULL,
[BRAND] [nvarchar](255) NULL,
[MODEL] [nvarchar](255) NULL,
[Shift] [nvarchar](255) NULL,
[AST_POINT] [nvarchar](255) NULL,
[SOURCE] [nvarchar](255) NULL,
[AREA] [nvarchar](255) NULL,
[POST DATE] [nvarchar](255) NULL,
[PRICE] [float] NULL,
[DISC] [float] NULL,
[Net Payment] [float] NULL
)
My query is like this:
WITH step1 AS (
SELECT [RESULT],[CLASSIFICATION_2],[ARMADA],[CDAREA],[BRAND],[MODEL],[Shift],[AST_POINT],[SOURCE],[AREA]
,CONVERT(DATE, [POST DATE]) AS [POST DATE],COUNT(*) AS Total
FROM [AstraWorldDB].[dbo].[ERA_Report2]
GROUP BY [RESULT],[CLASSIFICATION_2],[ARMADA],[CDAREA],[BRAND],[MODEL],[Shift],[AST_POINT],[SOURCE],[AREA]
,[POST DATE]
),
step2 AS (
SELECT [RESULT],[CLASSIFICATION_2],[ARMADA],[CDAREA],[BRAND],[MODEL],[Shift],[AST_POINT],[SOURCE],[AREA]
,[POST DATE],Total
,CASE
WHEN MONTH([POST DATE]) > DATEPART(MONTH, DATEADD(MONTH, -1, [POST DATE]))
THEN (SELECT COUNT(*)
FROM step1
WHERE DATEPART(MONTH, [POST DATE]) = DATEPART(MONTH, DATEADD(MONTH, -1, [POST DATE])) AND
DATEPART(YEAR, [POST DATE]) = DATEPART(YEAR, DATEADD(MONTH, -1, [POST DATE]))
)
ELSE '0'
END AS LastMonthTotal
FROM step1
)
SELECT *
FROM step2
WHERE AREA = 'DPS'
ORDER BY [POST DATE]
but it keeps returning 0 when it shouldn't. This is the result:
result
where POST DATE = 2020-02-01, LastMonthTotal must be 2.

Answer 1

In all supported versions of SQL Server, you would use LAG() -- assuming that every month has data:
select t.*,
lag(total) over (partition by [RESULT],[CLASSIFICATION_2], [ARMADA],[CDAREA], [BRAND], [MODEL], [Shift], [AST_POINT], [SOURCE], [AREA] order by [POST DATE]) as prev_month_total
from <your table> t;
This seems like a reasonable assumption.
In older versions, you would presumably use a join:
select t.*, tprev.total
from <your table> t left join
<your table> tprev
on tprev.result = t.result and . . . -- all the columns that define the group
tprev.post_date = dateadd(month, -1, t.post_date)