SQL 分组去重

select *
  from (select p.province_name,
               p.province_code,
               c.city_name,
               c.city_code,
               c.city_id,
               ROW_NUMBER() OVER(PARTITION BY p.province_name order BY c.city_id DESC) AS r
          from hs_basic_province p
          left join hs_basic_city c
            on c.province_id = p.province_id) t
 where t.r <3;

Oracle中row_number() over(partition by xxx order by xxx)的用法
row_number() over(partition by a order by b)
上面的意思就是将查询结果按照a字段分组（partition），
然后组内按照b字段排序，至于asc还是desc，可自行选择，
然后为每行记录返回一个rownumber用于标记顺序