How do I get x rows after ordering and grouping?

Hi:

      How do I get x number of rows in a select statement after 
ordering and grouping?

      I have the next select statement:

      SELECT * FROM SALES ORDER BY CREATION_DATE

      This statement give me many rows, but I only need de first 20 
rows, or only I need the rows 20 to 40, or only I need the last 20 rows, 
it depends.

      Are there any form to handle it?

THANKS


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