我想你可能不需要 distinct 在子查询或整个子查询中。

select p.user_id as id, sum(case when p.sale_type = 'cash' then p.amount else 0 end) as amount
from payments p 
where p.payment_time = '2018-04-01' 
group by p.user_id

或者没有 case ```
select p.user_id, sum(p.amount)
from payments p
where p.sale_type = 'cash' and p.payment_time = '2018-04-01'
group by p.user_id

你为什么不使用为这个目的而设的“having”子句呢。

SELECT SUM(amount) AS cash FROM payments
WHERE payment_time = '2018-04-01'
GROUP BY sale_type
HAVING sale_type= 'cash'

你需要加上 sale_type 列到 GROUP BY 内部查询的 group by user_id, sale_type 查询样式的正确结果。
p、 实际上，我认为你不需要子查询。
上面的查询给出的结果是 60 ，而

select SUM(CASE WHEN p1.sale_type='cash' THEN p1.amount ELSE 0 END) as cash
  from 
 (select distinct user_id, SUM(amount) AS amount, sale_type 
    from payments 
   where payment_time = date'2018-04-01' 
   group by user_id, sale_type) p1;

或者

select SUM(CASE WHEN sale_type='cash' THEN amount ELSE 0 END) as cash 
  from payments 
 where payment_time = date'2018-04-01'

给予 40 结果 cash 柱
sql fiddle演示