Data sampling in PostgreSQL

sampling_example.sql

-- sample data and assign new or returning user type
sampledata as (
select a.user_id, 
    a.created_at
, case 
    when a.created_at between a.firstdate and a.firstdate+27 then 'new' 
        else 'returning' 
        end as type
from (
    select user_id 
        , left(right(user_id, 4), 2) as sample
        , created_at
        , firstdate::date as first_date
    from data
    group by 1, 2, 3, 4
) a
where a.sample='04'
group by 1, 2
)