想象一下下表用户
Users
------------------------
CITY USERNAME TOPIC
city1 george topic1
city1 george topic2
city1 george topic3
city1 chris topic1
city1 chris topic2
city2 john topic1
city3 jenny topic1
我想要按城市划分的以下结果组
city1 -> 2 users
city2 -> 1 user
city3 -> 1 user
任何帮助表示赞赏!