单列索引、多列索引及如何选择合适的索引列顺序

1、单列索引
最常见的索引类型涉及单个列，在数据结构中存储该列的值的副本，允许快速查找具有相应列值的行。 B树数据结构允许索引在WHERE子句中快速查找特定值，一组值或一系列值，对应于运算符如=，>，≤，BETWEEN，IN等。

mysql> create table people(
    -> last_name varchar(50) not null,
    -> first_name varchar(50) not null,
    -> sex enum('m','f') not null,
    -> key last_name(last_name)			//单列索引
    -> )engine=innodb;
Query OK, 0 rows affected (0.07 sec)			
	
mysql> select * from people;
+-----------+------------+-----+
| last_name | first_name | sex |
+-----------+------------+-----+
| Akroyd    | Christian  | f   |
| Akroyd    | Lumi       | m   |
| Barrymore | Debbie     | m   |
| Barrymore | Kirsten    | m   |
| Cathy     | Kim        | m   |
+-----------+------------+-----+
5 rows in set (0.00 sec)

mysql> explain select * from people where last_name > 'Barrymore';		//使用了索引进行查找
+----+-------------+--------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
| id | select_type | table  | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                 |