Acceptance Rate By Date

What is the overall friend acceptance rate by date? Your output should have the rate of acceptances by the date the request was sent. Order by the earliest date to latest.
Assume that each friend request starts by a user sending (i.e., user_id_sender) a friend request to another user (i.e., user_id_receiver) that's logged in the table with action = 'sent'. If the request is accepted, the table logs action = 'accepted'. If the request is not accepted, no record of action = 'accepted' is logged.

Table: fb_friend_requests

fb_friend_requests
Preview
user_id_sender:
varchar
user_id_receiver:
varchar
date:
datetime
action:
varchar

--acceptance rate = # accepted / # sent

SELECT
    a.date,
    count(b.user_id_receiver)/count(a.user_id_sender) ::float as 
    acceptance_rate
FROM(
    SELECT 
        user_id_sender,
        user_id_receiver,
        date,
        action
    FROM fb_friend_requests
    WHERE action='sent') a

LEFT JOIN (
    SELECT 
        user_id_sender,
        user_id_receiver,
        date,
        action
    FROM fb_friend_requests
    WHERE action='accepted') b 
    on b.user_id_sender = a.user_id_sender
    and b.user_id_receiver= a.user_id_receiver 
GROUP BY a.date
ORDER BY a.date