Custom select query two tables by a meta key

First of all you are trying to combine both of the tables by post_parent.

$wpdb->posts.post_parent = $wpdb->ftcalendar_events.post_parent

I guess data in the ftcalendar_events has children of posts table, try 

$wpdb->posts.ID = $wpdb->ftcalendar_events.post_parent

Also you can try joining tables:

SELECT * FROM $wpdb->posts JOIN $wpdb->ftcalendar_events
ON $wpdb->posts.ID = $wpdb->ftcalendar_events.post_parent
WHERE $wpdb->posts.post_status="publish" AND $wpdb->posts.post_type="post"
ORDER BY $wpdb->posts.post_date DESC

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