mysql> select database();                                                           
+------------+
| database() |
+------------+
| sakila     |
+------------+
1 row in set (0.00 sec)

mysql> create table city_demo (city varchar(50) not null);                          
Query OK, 0 rows affected (0.02 sec)

mysql> insert into city_demo (city) select city from city;                          
Query OK, 600 rows affected (0.08 sec)
Records: 600  Duplicates: 0  Warnings: 0

mysql> insert into city_demo (city) select city from city_demo;
Query OK, 600 rows affected (0.07 sec)
Records: 600  Duplicates: 0  Warnings: 0

mysql> update city_demo set city = ( select city from city order by rand() limit 1);
Query OK, 1199 rows affected (0.95 sec)
Rows matched: 1200  Changed: 1199  Warnings: 0

mysql>

mysql> select count(*) as cnt, city from city_demo group by city order by cnt desc limit 10;               
+-----+--------------+
| cnt | city         |
+-----+--------------+
|   8 | Garden Grove |
|   7 | Escobar      |
|   7 | Emeishan     |
|   6 | Amroha       |
|   6 | Tegal        |
|   6 | Lancaster    |
|   6 | Jelets       |
|   6 | Ambattur     |
|   6 | Yingkou      |
|   6 | Monclova     |
+-----+--------------+
10 rows in set (0.01 sec)

mysql>

mysql> select count(*) as cnt,left(city,3) as pref from city_demo group by pref order by cnt desc limit 10;
+-----+------+
| cnt | pref |
+-----+------+
|  25 | San  |
|  15 | Cha  |
|  12 | Bat  |
|  12 | Tan  |
|  11 | al-  |
|  11 | Gar  |
|  11 | Yin  |
|  10 | Kan  |
|  10 | Sou  |
|  10 | Bra  |
+-----+------+
10 rows in set (0.00 sec)

mysql> select count(*) as cnt,left(city,4) as pref from city_demo group by pref order by cnt desc limit 10; 
+-----+------+
| cnt | pref |
+-----+------+
|  12 | San  |
|  10 | Sout |
|   8 | Chan |
|   8 | Sant |
|   8 | Gard |
|   7 | Emei |
|   7 | Esco |
|   6 | Ying |
|   6 | Amro |
|   6 | Lanc |
+-----+------+
10 rows in set (0.01 sec)

mysql> select count(*) as cnt,left(city,5) as pref from city_demo group by pref order by cnt desc limit 10; 
+-----+-------+
| cnt | pref  |
+-----+-------+
|  10 | South |
|   8 | Garde |
|   7 | Emeis |
|   7 | Escob |
|   6 | Amroh |
|   6 | Yingk |
|   6 | Moncl |
|   6 | Lanca |
|   6 | Jelet |
|   6 | Tegal |
+-----+-------+
10 rows in set (0.01 sec)

mysql> select count(*) as cnt,left(city,6) as pref from city_demo group by pref order by cnt desc limit 10; 
+-----+--------+
| cnt | pref   |
+-----+--------+
|   8 | Garden |
|   7 | Emeish |
|   7 | Escoba |
|   6 | Amroha |
|   6 | Yingko |
|   6 | Lancas |
|   6 | Jelets |
|   6 | Tegal  |
|   6 | Monclo |
|   6 | Ambatt |
+-----+--------+
10 rows in set (0.00 sec)

mysql>

mysql> select count(distinct city) / count(*) from city_demo;
+---------------------------------+
| count(distinct city) / count(*) |
+---------------------------------+
|                          0.4283 |
+---------------------------------+
1 row in set (0.05 sec)

mysql>

mysql> 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 
    -> from city_demo;
+--------+--------+--------+--------+
| sel3   | sel4   | sel5   | sel6   |
+--------+--------+--------+--------+
| 0.3367 | 0.4075 | 0.4208 | 0.4267 |
+--------+--------+--------+--------+
1 row in set (0.01 sec)

mysql>

mysql> alter table city_demo add key (city(6));
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>

mysql> explain select * from city_demo where city like 'Jinch%';
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | city_demo | range | city          | city | 20      | NULL |    2 | Using where |
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql>