Get each 7th record

I have a statistical table in PostgreSQL table:

开发者_Python百科article_id | date       | read_count
    1      | 2011-06-02 | 12
    1      | 2011-06-03 | 54
    1      | 2011-06-04 | 2
    1      | 2011-06-05 | 432

And as i need this data in a chart, i need to get values in each week.

I know Postgres has a row_number() function, anyway i didn't get it to work properly.

SELECT "date", "read_count" FROM "articles_stats" 
WHERE row_number() OVER (ORDER BY "date" ASC) % 7 = 0
ORDER BY "date" ASC

ERROR: window functions not allowed in WHERE clause

From @pst's link,

"If there is a need to filter or group rows after the window calculations are performed, you can use a sub-select."

For example:

  SELECT * 
    FROM (
          SELECT "date", "read_count", 
                 row_number() OVER (ORDER BY "date" ASC) as n
            FROM "articles_stats" 
         ) x
   WHERE x.n % 7 = 0
ORDER BY x."date" ASC