可以使用窗口函数：

select s.*,
       (case when row_number() over (partition by user_id order by s_date) = 1
             then 'Yes' else 'No'
        end) as new_customer
from sales s;

然后可以为摘要聚合：

select s_date, sum(case when new_customer = 'Yes' then 1 else 0 end)
from (select s.*,
             (case when row_number() over (partition by user_id order by s_date) = 1
                   then 'Yes' else 'No'
              end) as new_customer
      from sales s
     ) s
group by s_date