How can I add a calendar as a column/field to a view table?

I need to add another calendar to the current view table. The table already includes a column for month and year, but some months have no transactions, so they don't show up in the table. I want to create a new calendar field to display monthly sales, regardless of whether there are any transactions. How would I do this?

Sample script:

select  T.MONTH_YEAR,
        ROUND(SUM(T.GROSS_AMOUNT),3) AS CURRENT_GROSS_AMOUNT,
        lag(ROUND(SUM(T.GROSS_AMOUNT),3)) over (partition by T.MATERIAL order by min(T.MONTH_YEAR)) as PREV_GROSS_AMOUNT
from transactions T

Here's the output:

MONTH_YEAR	CURRENT_GROSS_AMOUNT	PREV_GROSS_AMOUNT
Jan 2024	200
Feb 2024	200	200
Mar 2024	300	200
Apr 2024	400	300
Jul 2024	500	400
Aug 2024	600	500

The final script should be something like this:

select  T.MONTH_YEAR,
        /*add another calendar to display 2 months no display */
        ROUND(SUM(T.GROSS_AMOUNT),3) AS CURRENT_GROSS_AMOUNT,
        lag(ROUND(SUM(T.GROSS_AMOUNT),3)) over (partition by T.MATERIAL  order by min(T.MONTH_YEAR)) as PREV_GROSS_AMOUNT
from transactions T

Which would output:

MONTH_YEAR	calendar date	CURRENT_GROSS_AMOUNT	PREV_GROSS_AMOUNT
Jan 2024	Jan 2024	200
Feb 2024	Feb 2024	200	200
Mar 2024	Mar 2024	300	200
Apr 2024	Apr 2024	400	300
	May 2024	0	400
	Jun 2024	0	0
Jul 2024	Jul 2024	500	0
Aug 2024	Aug 2024	600	500

Solution

All you need is :

FROM
    UNNEST(GENERATE_DATE_ARRAY('2024-01-01', '2024-12-31', INTERVAL 1 MONTH)) AS month_date

Step 1: Create a calendar table [ you can change the date range as per your need]. This is the part which you basically need in your code the rest you can change as you wish.
Step 2: Aggregate transaction data by month (what you have done so far)
Step 3: Left Join the calendar with transaction data, this will join the data together and still show the calendar dates which are not being used for any transaction


WITH calendar AS (
  SELECT
    FORMAT_DATE('%b %Y', month_date) AS month_year,
    month_date
  FROM
    UNNEST(GENERATE_DATE_ARRAY('2024-01-01', '2024-12-31', INTERVAL 1 MONTH)) AS month_date
),


transaction_data AS (
  SELECT
    FORMAT_DATE('%b %Y', PARSE_DATE('%b %Y', T.month_year)) AS month_year,
    ROUND(SUM(T.gross_amount), 3) AS current_gross_amount
  FROM
    transactions T
  GROUP BY
    month_year
)

SELECT
  c.month_year,
  IFNULL(td.current_gross_amount, 0) AS current_gross_amount,
  LAG(IFNULL(td.current_gross_amount, 0)) OVER (ORDER BY c.month_date) AS prev_gross_amount
FROM
  calendar c
LEFT JOIN
  transaction_data td
ON
  c.month_year = td.month_year
ORDER BY
  c.month_date