Syntax

• select round(val, digit)

friend-requests-i-overall-acceptance-rate

select
round(
    ifnull(
(select count(*) from (select distinct requester_id, accepter_id from request_accepted ) as request)
    /        
(select count(*) from (select distinct sender_id, send_to_id from friend_request ) as sender ),
  0),
2) as accept_rate;        

• Self Join

SELECT column_name(s)
FROM table1 T1, table1 T2
WHERE condition;