找出有多少用户在上面有余额£在过去的30天里至少有一次，
您可以使用窗口函数来计算每个用户在此期间的运行余额。然后，您只需统计运行平衡曾经超过阈值的不同用户：

select count(distinct user_id)  no_users
from (
    select 
        user_id,
        sum(case when type = 'credit' then amount else -amount end) 
            over(partition by user_id order by created_at) balance
    from transaction
    where created_at >= current_date - interval '30' day and created_at < current_date
) t
where balance > 2000

使用条件聚合：

select user_id,
       (sum(amount) filter (where type = 'credit') -
        coalesce(sum(amount) filter (where type = 'debit'), 0)
       )
from public.transaction t
where t.created_at >= CURRENT_DATE - INTERVAL '30 days' and
      t.created_at < CURRENT_DATE
group by user_id;

SELECT user_id, 
   c.credit_amount - b.debit_amount AS balance
FROM public.transaction a
JOIN (SELECT 
    user_id, type, amount AS debit_amount, 
  FROM public.transaction 
  where a.type LIKE 'debit') b on a.user_id = b.user_id
JOIN (SELECT
  user_id, type, amount AS credit_amount
  FROM public.transaction 
  where type LIKE 'credit') c on a.user_id = c.user_id
WHERE a.created_at >= CURRENT_DATE - INTERVAL '30 days'
AND a.created_at <= CURRENT_DATE) AS table_1
AND (c.credit_amount - b.debit_amount) > 2000
GROUP BY a.user_id;