You want the difference, in days, between the date of each record and the date of the record before the last downgrade.
When you have a specific record, the record before the last downgrade is the record that
- has a higher rating than this record, and
- has a lower date than this record, and
- is the latest record of those.
In SQL, this can be done with a correlated subquery:
SELECT date,
rating,
(SELECT date
FROM MyTable AS downgrade
WHERE downgrade.date < MyTable.date
AND downgrade.rating < MyTable.rating
ORDER BY date DESC
LIMIT 1) AS downgrade_date
FROM MyTable
date rating downgrade_date
---------- ---------- ----------
2010-03-04 A
2010-03-05 B 2010-03-04
2010-03-06 B 2010-03-04
2010-03-07 B 2010-03-04
2010-03-08 B 2010-03-04
2010-03-09 C 2010-03-08
2010-03-10 C 2010-03-08
To compute the difference, convert the date into a numeric value. You can then use this value for further computations:
SELECT date,
rating,
CASE
WHEN days <= 3 THEN days
ELSE 0
END AS whatever
FROM (SELECT date,
rating,
julianday(date) -
julianday((SELECT date
FROM MyTable AS downgrade
WHERE downgrade.date < MyTable.date
AND downgrade.rating < MyTable.rating
ORDER BY date DESC
LIMIT 1)) AS days
FROM MyTable)