sql calculate retention

Question

how do I calculate retention rate in SQL to add as a column to my table

1. first day I had in Brazil 411 unique users
2. second day only 154 users came back (retention rate is 154/411)
3. third day only 115 came back again (115/154)

4. last day only 81 out of 98 users came back

     Country Date sum_views distinct_user_ids Brazil 5/5/2016 3793 411 Brazil 5/6/2016 1632 154 Brazil 5/7/2016 1456 115 Brazil 5/8/2016 1223 98 Brazil 5/9/2016 993 81 Canada 5/5/2016 6419 708 Canada 5/6/2016 2649 235 Canada 5/7/2016 2578 197 Canada 5/8/2016 2024 151 Canada 5/9/2016 1893 141 United states 5/5/2016 13007 1438 United states 5/6/2016 5755 522 United states 5/7/2016 5502 419 United states 5/8/2016 4915 362 United states 5/9/2016 3713 284 

output :

a column with retention rate

Answer 1

You need the previous value for the denominator. You can get that with a correlated subquery:

select t.*, tprev.distinct_user_ids,
       (t.distinct_user_ids / tprev.distinct_user_ids)
from t join
     t tprev
     on tprev.country = t.country and
        tprev.date = date(t.date, '-1 day');