SQL 计算跳出率

Question

I have the following SQL table (Postgres):

+-----------+------------+-------+
| SessionID |  Received  | Event |
+-----------+------------+-------+
|         1 | 1595207019 | visit |
|         1 | 1595207020 | play  |
|         2 | 1595207040 | visit |
|         1 | 1595207050 | click |
+-----------+------------+-------+

I want to calculate bounce rate where bounce is defined as visit event not followed by any other event with the same session id.

Answer 1

Hmmm. . . I think you want to summarize by session_id and count the types of visits. then aggregate. Your question is not 100% clear, but I thinK:

select (count(*) filter (where num_notvisits = 0) * 1.0 / count(*)) as bounce_rate
from (select session_id,
             count(*) filter (where event = 'visit') as num_visits,
             count(*) filter (where event <> 'visit') as num_notvisits
      from t
      group by session_id
     ) s
where num_visits > 0;

This is the ratio of the number of sessions with both a visit and a non-visit event divided by the number of sessions with a visit.

You can actually phrase the outer select more simply as:

select avg( (num_notvisits = 0)::int ) as bounce_rate

Answer 2

You can explicitly query for the number of "bounces" like this:

select count(*)
from t as t1
where t1.event = 'visit'
  and not exists (select * from t as t2 where t1.received < t2.received and t1.sessionid = t2.sessionid)

Not sure what is the denominator of the "bounce rate" specifically? Bounces per session? # bounces / # events?