Add missing months with values from previous month - sql

Question

I need to use this SQL query for a software and get the time in a particular format hence the reason for the Time column however I need the query to insert the months that are missing with the value from the previous month. This is the query I currently have.
SELECT [accountnumber],SUM([postingamount]) AS Amount, [accountingdate],
convert(varchar(4),year(accountingdate))+'M'+ Format(DATEPART( MONTH, accountingdate) , '00')
AS [Time]
FROM [7 GL Detail MACL]
where [accountingdate]>='2019-01-01'
GROUP BY [accountingdate],[postingamount],[accountnumber]
Current Results
Expected Results

Answer 1

Since you didn't specify the RDBMS system you're using, I can't guarantee that this logic will work because every system uses slightly different SQL syntax.
However I used Rasgo datespine function to generate this SQL, as it is quite complex to wrap your head around, and tested it on Snowflake.
The main differences between Snowflake and other systems are: DATEADD and TABLE (GENERATOR())
In case you can't modify this to work in your system, here are the basic steps which you'll want to follow:
Select unique accountnumbers
Select unique dates (month beginnings?) This is where Snowflake uses GENERATOR but other systems might actually have a Calendar table you can select from
Cross Join (cartesian join) these to create every possible combination of accountnumber and date
Outer Join #3 to your data (might have to truncate your date to month-begin)
Filter out rows that dont apply. Like for instance you might have just inserted a row for 1/1/2019 for an account that didn't even begin until 12/12/2020.
WITH GLOBAL_SPINE AS (
SELECT
ROW_NUMBER() OVER (ORDER BY NULL) as INTERVAL_ID,
DATEADD('MONTH', (INTERVAL_ID - 1), '2019-01-01'::timestamp_ntz) as SPINE_START,
DATEADD('MONTH', INTERVAL_ID, '2022-06-01'::timestamp_ntz) as SPINE_END
FROM TABLE (GENERATOR(ROWCOUNT => 42))
),
GROUPS AS (
SELECT
accountnumber,
MIN(DESIRED_INTERVAL) AS LOCAL_START,
MAX(DESIRED_INTERVAL) AS LOCAL_END
FROM [7 GL Detail MACL]
GROUP BY
accountnumber
),
GROUP_SPINE AS (
SELECT
accountnumber,
SPINE_START AS GROUP_START,
SPINE_END AS GROUP_END
FROM GROUPS G
CROSS JOIN LATERAL (
SELECT
SPINE_START, SPINE_END
FROM GLOBAL_SPINE S
WHERE S.SPINE_START >= G.LOCAL_START
)
)
SELECT
G.accountnumber AS GROUP_BY_accountnumber,
GROUP_START,
GROUP_END,
T.*
FROM GROUP_SPINE G
LEFT JOIN {{ your_table }} T
ON DESIRED_INTERVAL >= G.GROUP_START
AND DESIRED_INTERVAL < G.GROUP_END
AND G.accountnumber = T.accountnumber;
You were also doing an aggregation step, but I figure once you get this complicated part down, you can figure out how to finally aggregate it the way you want it.