create table city_demo(city varchar(50) not null);

insert into city_demo(city) select city from city

insert into city_demo(city) select city from city_demo

update city_demo set city=(select city from city ORDER BY rand() limit 1)

select * from city;

select * from city_demo;

select count(*) as cnt,city  from city_demo group by city order by cnt desc limit 10

select count(*) as cnt,left(city,3) as pref  from city_demo group by pref order by cnt desc limit 10

select count(*) as cnt,left(city,7) as pref  from city_demo group by pref order by cnt desc limit 10

select count(DISTINCT city)/count(*) from city_demo

select count(distinct city),city from city_demo  -- Athenai

select count(distinct left(city,3))/count(*) as sel3,

count(distinct left(city,4))/count(*) as sel4,

count(distinct left(city,5))/count(*) as sel5,

count(distinct left(city,6))/count(*) as sel6,

count(distinct left(city,7))/count(*) as sel7,

count(distinct left(city,8))/count(*) as sel8  from city_demo  -- 接近0.031