Sql random sampling per group

with randomly_sorted_users as (

    select
        user_id,
        signup_date,
        row_number() over(partition by date_trunc('year', signup_date)
                            order by random()) as random_sort
    from
        user_table

)
select
    user_id,
    signup_date
from
    randomly_sorted_users
where
    random_sort <= 5

Source: stackoverflow.com

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