How to get a column of the select to be "cumulative" (like in excel)?
If I understand you correctly, you want to have another field in your query which shows the total count for all posts, regardless of dates then.
You could achieve this like so :

 SELECT DATE(post_date) AS date, 
           concat(post_title) titles, COUNT(*) AS count, 
           (select count(*) from wp_posts 
            WHERE post_status = 'publish'    
            AND post_parent = 0
            AND post_type = 'post') as CumulativeCount
    FROM  wp_posts 
    WHERE post_status = 'publish'
    AND post_parent = 0
    AND post_type = 'post'
    GROUP BY DATE(post_date)
    ORDER BY DATE(post_date)