Running total over date range - fill in the missing dates

Question

I have the following table.

DATE  | AMT
10/10 | 300
12/10 | 300
01/11 | 200
03/11 | 100

How do I get the monthly total? A result like -

Answer 1

To cater for missing months, create a template table to join against.

Think of it as caching. Rather than looping through and filling gaps, just have a calendar cached in your database.

You can even combine multiple calendars (start of month, start of week, bank holidays, working day, etc) all into one table, with a bunch of search flags and indexes.

You end up with something like...

SELECT
  calendar.date,
  SUM(data.amt)
FROM
  calendar
LEFT JOIN
  data
    ON  data.date >= calendar.date
    AND data.date <  calendar.date + INTERVAL 1 MONTH
WHERE
      calendar.date >= '20110101'
  AND calendar.date <  '20120101'
GROUP BY
  calendar.date

EDIT

I just noticed that the OP wants a running total.

This -is- possible in SQL but it is extremely inefficient. The reason being that the result from one month isn't used to calculate the following month. Instead the whole running-total has to be calculated again.

For this reason It is normally strongly recommended that you calculate the monthly total as above, then use your application to itterate through and make the running total values.

If you really must do it in SQL, it would be something like...

SELECT
  calendar.date,
  SUM(data.amt)
FROM
  calendar
LEFT JOIN
  data
    ON  data.date >= @yourFirstDate
    AND data.date <  calendar.date + INTERVAL 1 MONTH
WHERE
      calendar.date >= @yourFirstDate
  AND calendar.date <  @yourLastDate
GROUP BY
  calendar.date