Top N Ranking

In Bex I have a query that list top N customers with their ranking. In the same query I have a column with these top N customers's ranking for the same period last year. I need the customers that fell of the top 20 ranking of last year. Ranking last year is between 1 and 20 and this year not. I've tried a condition that works looking at both rankings, but when I active the condition, I don't get the result I'm looking for and it changes the ranking.