why mysql not using index when that index is based on a varchar field?

[Zhixin Gao]

Say, I have a table that is:

create table test (
id int primary key auto_increment,
c_no varchar(11),
c2 varchar(2),

key c_no(c_no)
);
Then based on this table, I have a sql query which is:

select
*
from
test
where
c_no = 100
But when I used explain to analyze the sql, I found it didn't use the index on c_no:

explain select * from test2 where c_no=100


+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ALL | c_no | NULL | NULL | NULL | 7 | 14.29 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)
why?