SQL累计支付金额前20%

交易表：user_sales_table
字段：  user_name pay_amount(支付金额)
需求：求累计支付金额前20%

select user_name from (
    select user_name,ntile(5) over(order by tpay desc) nt from (
        select user_name,sum(pay_amount) tpay from user_sales_table group by user_name
    ) t1  
) t2 where t2.nt = 1