如何减去前一年，但在MySQL中增加月份？

Question

I am new to procedural programming and mysql, but suppose I have a function PrevMonth(in_date date, in_mn_count int) The first is a date and the second is an integer which is expected to be the number of months. The function returns a string with the format 'YYYY-MM' which is the year and month, where the year in the first parameter is the previous year, and in_mn_count is added to in_date.

For example, a_testbed.PrevMonth('2012-05-19', 6) returns '2011-11'

This is what I have:

set return_date := Date_format(DATE_ADD(in_date, INTERVAL -1 YEAR),
DATE_ADD(in_date,    INTERVAL in_mn_count MONTH),'%Y %M'); 

Answer 1

You have several issues here, your format for DATE_FORMAT is wrong, and you are passing the date through DATE_ADD twice for some odd reason.

DATE_FORMAT(DATE_ADD(in_date, INTERVAL (in_mn_count-12) MONTH), '%Y-%m')

That would do the trick, because 12 months is 1 year.

Note the small 'm' in '%Y-%m', that is actually what you want if you desire '2011-05' type of return value.