你需要一些窗口函数加上这样的 CASE：

case 
       -- If reason code is starts with "RQT"
  when first_value(Rscode) over (partition by custid order by enter_date) = 'RQT'
       -- and any of the valid code is followed By RQT
   and lead(Rscode) over (partition by custid order by enter_date) = 'RQT' then 'No'

       -- If two dates difference is <=6 moths and reason code is valid then new column value should be "Yes" other wise "No"
  when enter_date <= add_months(lag(enter_date) over (partition by custid order by enter_date),6)
   and valid = 'Yes'
    then 'Yes'

  else 'No' -- also covers: For first row,New column value should be always "No"
end