monthly_stats.msql

select m.month, coalesce(s.signup_count, 0), coalesce(p.purchase_count, 0)
from (
    select extract(month from ts) as month
    from signups
    union all
    select extract(month from ts) as month
    from purchases
    group by month
) as m
left outer join (
  select extract(month from ts) as month, count(user_id) as signup_count
  from signups
  group by extract(month from ts)
) s
on m.month = s.month
left outer join (
  select extract(month from ts) as month, count(purchase_id) as purchase_count
  from purchases
  where product_id not in (2342, 8980)
  group by extract(month from ts)
) p
on m.month = p.month;