两个开窗函数，一个求最小日期，一个求数量，然后你会发现重复了，所以加个distinct

 select p.* from (
   select DISTINCT user_id,min(date) over(partition by user_id order by date asc) as first_buy_date
          ,count(*) over(partition by user_id) as cnt
    from order_info
    where status='completed' and date>'2025-10-15' 
    and product_name in ('C++','Java','Python')
 ) p
 where p.cnt>1
 order by p.user_id asc