select distinct(t.area) ,(select count(1) as num from aa s where t.area=s.area) from aa t

------解决方案--------------------
C# code

string oraclesql;
            string sql = "select country,count(*) num from aa where area in({0})";
            StringBuilder sb = new StringBuilder();
            string[] c = { "延庆县", "怀柔区", "密云县", "昌平区", "顺义区", "平谷区", "门头沟区", "北京市", "通州区", "虎山区", "大兴区" };
            foreach (string s in c)
            {
                sb.Append(string.Format("'{0}',", s));
            }
            oraclesql = string.Format(sql, sb.ToString().TrimEnd(','));
            //用oraclesql在数据库中执行