Fill in gaps in cumulative query - oracle

Question

I have the following query to calculate a cumulative sum:
SELECT to_char(INVC_DT, 'MON-YYYY') AS MONTH,
SUM(INVC_AMT),
SUM(SUM(INVC_AMT)) OVER (ORDER BY MIN(INVC_DT))
FROM T_INVC_INFO I INNER JOIN T_TASK_INFO T ON I.TASK_ID = T.TASK_ID
WHERE T.CNTRCT_ID = #session.user.cntrct_id#
GROUP BY to_char(INVC_DT, 'MON-YYYY')
ORDER BY MONTH DESC;
The problem I have is that I need to fill in the gaps in time. For example, if I have data for JAN, FEB, and APR, I need another row for MAR with value 0.

Answer 1

You may introduce a calendar table into your query, which will represent every month/year which you want to appear in your output. Assuming you wanted to cover all of 2017 to 2019, you might try:
WITH years AS (
SELECT '2017' AS year FROM dual UNION ALL
SELECT '2018' FROM dual UNION ALL
SELECT '2019' FROM dual
),
months AS (
SELECT 'JAN' AS month FROM dual UNION ALL
SELECT 'FEB' FROM dual UNION ALL
...
SELECT 'DEC' FROM dual
)
SELECT
m.month || '-' || y.year AS MONTH,
SUM(INVC_AMT),
SUM(SUM(INVC_AMT)) OVER (ORDER BY MIN(INVC_DT))
FROM years y
CROSS JOIN months m
LEFT JOIN T_INVC_INFO i
ON m.month || '-' || y.year = TO_CHAR(i.INVC_DT, 'MON-YYYY')
LEFT JOIN T_TASK_INFO t
ON i.TASK_ID = t.TASK_ID AND
t.CNTRCT_ID = #session.user.cntrct_id#
WHERE
TO_DATE(m.month || '-' || y.year, 'MON-YYYY') BETWEEN
(SELECT MIN(INVC_DT) FROM T_INVC_INFO) AND
(SELECT MAX(INVC_DT) FROM T_INVC_INFO)
GROUP BY
m.month || '-' || y.year
ORDER BY
MONTH DESC;