How to get a column of the select to be "cumulative" (like in excel)? This simple query (written in MySQL) is counting the number of WP posts published every day. But I need another information. I want to know the cumulative number of post for each date returned by the query.

If there are 100 posts published on this blog at date X I want the query to show 100. If I publish another post at date X + 1, I want to read X+1 => 101 posts

How can I achieve this objective?

SELECT DATE(post_date)    AS date, 
       concat(post_title) AS titles, 
       COUNT(*)           AS count
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)
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)