If your Month field was created with the month() function from a date it will have both a text and a numeric value. So try this:
=sum({$<Month = {">=$(=only(Month)-1)<=$(=only(Month)-1)"}>} 金额 )
One would think that if the above works then something like Month={"=5"} would work too, but it doesn't. Yet Month={">=5<=5"} does. I hope QlikTech comes up with a saner way to deal with dual value fields in set analysis expressions soon.
If your Month only has a text value, then use the dual() function to give it both a text and numeric value or create a numeric field like for example MonthNum too. Better yet, use the Qlikview Components library to generate a calendar for you with all sorts of handy fields and set expressions.
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