title: An interesting SQL query CHALLENGE - list the AVERAGE value of the top 3 scores of each athlete
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  <pre>
  An interesting SQL query CHALLENGE:
  
  A table named athelets consisting of id, ath_id, name, score, date.
  
  +----+--------+-----------------+--------+------------+
  | id | ath_id | name            | record | r_date     |
  +----+--------+-----------------+--------+------------+
  |  1 |      2 | John Wayne      |     79 | 2010-07-08 |
  |  2 |      7 | Ronald Regan    |     51 | 2000-03-22 |
  |  3 |      1 | Ford Harrison   |     85 | 2009-11-13 |
  |  4 |      2 | John Wayne      |     69 | 2017-01-01 |
  
  Please write a sql query to list the average value of the top three scores of each athlete, something like:
  
  ath_id: 1, the arithmetic mean of his/her top 3 records: 77
  ath_id: 2, the arithmetic mean of his/her top 3 records: 73
  ath_id: 3, the arithmetic mean of his/her top 3 records: 47
  </pre>