通过较大查询中的最新行限制SQL查询

I'm generally pretty self reliant on fudging something together that works but I have run into a brick wall on this one and am eventually reaching out for a nod in teh right direction..

my query:

$post_views = (int)$wpdb->get_var("
        SELECT SUM(count) AS views
        FROM ".$wpdb->prefix."post_views
        WHERE id IN (".$post_id.") AND type = 0"

The database table looks like this :

id      type    period      count 
------- ------- ----------- -------
32310   0       20141023    8
32310   0       20141022    68
32310   1       201443      76
32310   2       201410      76
32310   3       2014        76
32310   4       total       76

The type 0 are the ones I'm interested in, I just want the sum of the COUNT column for the most recent 7 type 0 entries

I have been trying with things based around "ORDER BY period DESC LIMIT 7 " on the end of the query - to no avail, I generally get returns of 0 doing this.

a new type 0 row will be generated for each article every day, so thats why I need to only get the last 7

any help here would be massively appreciated, totally stuck for the first time ever with this.

SELECT SUM(count)
FROM (SELECT count
      FROM wp_post_views
      WHERE type = 0
        AND id IN (684,42,7)
      ORDER BY period DESC
      LIMIT 7)

Or just determine the date a week ago first and use that to filter, but a subquery like this will work fine as well.