题解 | #when按城市排序,如果城市为空，则按国家排序#

#两种方式：if和when

#if
#select customer_id,gender,city,country,age,latest_place_order_date from `customers_info` order by (if(city is not null,city,country))

#when
select customer_id,gender,city,country,age,latest_place_order_date from `customers_info` order by (case
    when city is null then country
    else city 
  end)