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Select every nth record from table
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Chronological Hierarchical 
~~~
SELECT * 
  FROM (
        SELECT ROW_NUMBER() OVER (PARTITION BY 1) RowNum, *
          FROM t
       ) AS A
 WHERE (A.RowNum % n) = 0
;

~~~
Edit: You need a version supporting Window functions, I think 3.26+ (unconfirmed)