To find "same-day" acceptance rate:
Select date,
count_accepted/count_sent as acceptance_rate from
(Select date, count(date) as count_accepted from
(Select * From table
where Action=’sent’
inner join (Select * From table where Action=‘accepted’) accepted on
accepted.User_id_who_sent=table.User_id_who_sent and
accepted.User_id_to_whom=table.User_id_to_whom and
accepted.date=table.date)
Group by date) t_of_accepted
Inner join
(Select date, count(date) as count_accepted
from table
where Action=’sent’
group by date) t_of_sent
On t_of_accepted.date=t_of_sent.date;