1、什么是较对集合:字符集的字符比较规则（collation,collate），一个字符集有多个较对集合。
mysql> create table ss (id int primary key auto_increment,
-> cc char(10));
insert into ss values (null,"a");
insert into ss values (null,"A");
insert into ss values (null,"b");
insert into ss values (null,"B");
-------------------------------------------------------------
mysql> select * from ss order by cc asc;
+----+------+
| id | cc |
+----+------+
| 1 | a |
| 2 | A |
| 3 | b |
| 4 | B |
+----+------+
mysql> select * from ss order by cc desc;
+----+------+
| id | cc |
+----+------+
| 3 | b |
| 4 | B |
| 1 | a |
| 2 | A |