Question

week      cookie
1         a
1         b
1         c
1         d
2         a 
2         b
3         a
3         c
3         d

This table represent someone visits a website in a particular week. Each cookie represents an individual person. Each entry represent someone visit this site in a particular week. For example, the last entry means ‘d’ come to the site in week 3.

I want to find out how many (same) people keep coming back in the following week, when given a start week to look at.

For example, if I look at week 1. I will get result like:

1 | 4
2 | 2
3 | 1

Because 4 user came in week 1. Only 2 of them (a,b) came back in week 2. Only 1 (a) of them came in all of these 3 weeks.

How can I do a select query to find out? The table will be big: there might be 100 weeks, so I want to find the right way to do it.

Answer 1

This query uses variables to track adjacent weeks and work out if they are consecutive:

set @start_week = 2, @week := 0, @conseq := 0, @cookie:='';
select conseq_weeks, count(*)
from (
select 
  cookie,
  if (cookie != @cookie or week != @week + 1, @conseq := 0, @conseq := @conseq + 1) + 1 as conseq_weeks,
  (cookie != @cookie and week <= @start_week) or (cookie = @cookie and week = @week + 1) as conseq,
  @cookie := cookie as lastcookie,
  @week := week as lastweek
from (select week, cookie from webhist where week >= @start_week order by 2, 1) x
) y
where conseq
group by 1;

This is for week 2. For another week, change the start_week variable at the top.

Here’s the test:

create table webhist(week int, cookie char);
insert into webhist values (1, 'a'), (1, 'b'), (1, 'c'), (1, 'd'), (2, 'a'), (2, 'b'), (3, 'a'), (3, 'c'), (3, 'd');

Output of above query with where week >= 1:

+--------------+----------+
| conseq_weeks | count(*) |
+--------------+----------+
|            1 |        4 |
|            2 |        2 |
|            3 |        1 |
+--------------+----------+

Output of above query with where week >= 2:

+--------------+----------+
| conseq_weeks | count(*) |
+--------------+----------+
|            1 |        2 |
|            2 |        1 |
+--------------+----------+

p.s. Good question, but a bit of a ball-breaker