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How long in days is a MONTH in MySQL?
If one finds where the past number of months equals the past number of days like this,
select distinct DATE_SUB(now(), INTERVAL 92 DAY),DATE_SUB(now(), INTERVAL 3 MONTH)
The numbers that are currently returned are,
- 1 month = 31 days
- 2 month = 62 days
- 3 month = 92 days
- 4 month = 123 days
- 5 month = 153 days
- 6 month = 184 days
Is MySQL rounding the number of days or is it taking into account the number of days in a particular month?
1 answer
There is no fixed number of days in a MONTH interval. DATE_SUB is mostly just decrementing the number in the months position of the date provided. So DATE_SUB('2020-09-14', INTERVAL 3 MONTH) is '2020-06-14' and DATE_SUB('2020-03-14', INTERVAL 3 MONTH) is '2019-12-14'. The documentation describes what happens in edge cases like DATE_SUB('2020-10-31', INTERVAL 1 MONTH):
If you add
MONTH,YEAR_MONTH, orYEARand the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month
So the result of DATE_SUB('2020-10-31', INTERVAL 1 MONTH) would be adjusted to '2020-09-30'.
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