date problem

To do date arithmetic on the field, you need to turn it into a date

Follow this code

declare @YYYYMM varchar(6)
set @YYYYMM = '200601'

select 
 convert(datetime, @YYYYMM+'01', 112),
 dateadd(mm, -1, convert(datetime, @YYYYMM+'01', 112)),
 convert(varchar, dateadd(mm, -1, convert(datetime, @YYYYMM+'01', 112)),112),
 left(convert(varchar, dateadd(mm, -1, convert(datetime, @YYYYMM+'01', 112)),112),6)

I first string concatenate a '01' to make a string that matches the date format of 112 (ISO standard). Then I can convert that to an actual date, then subtract one month using DATEADD, convert it back to a string from the 112 style date, and finally remove the last 2 characters to get your YYYYMM format back again.

The last one is the one you need, but I've shown the intermediary steps to help you understand the path....