索引（index）是对数据库表中一列或多列的值进行排序的一种结构，使用索引可快速访问数据库表中的特定

有这样的一个student 表

mysql> select * from student;
+----+-----+--------+---------------+------+-------+-----------+
| id | age | name   | register_date | sex  | grade | ManagerId |
+----+-----+--------+---------------+------+-------+-----------+
|  1 |  32 | alex   | 2018-10-17    | NULL |    88 | 3         |
|  4 |  34 | clause | 2018-08-22    | NULL |    98 | NULL      |
|  5 |  44 | alex   | 2018-08-22    | NULL |    68 | NULL      |
|  7 |  23 | VVVV   | 2018-10-29    | NULL |    80 | NULL      |
|  8 |  23 | asd    | 2018-10-29    | NULL |    82 | NULL      |
|  9 |  44 | alex   | 2019-01-02    | NULL |  NULL | NULL      |
| 18 | 100 | asd    | NULL          | NULL |  NULL | NULL      |
| 19 |  22 | alex   | NULL          | NULL |  NULL | NULL      |
| 20 | 102 | alex   | NULL          | NULL |  NULL | NULL      |
+----+-----+--------+---------------+------+-------+-----------+
9 rows in set (0.00 sec)

　为这样的一个表的name加一个索引

create index ind_sd1 on student(name)

查询其中名字为asd的信息

mysql> EXPLAIN select * from student where name='asd';
+----+-------------+---------+------------+------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | student | NULL       | ref  | ind_sd1       | ind_sd1 | 32      | const |    2 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

当建立索引时，遍历次数为2

取消索引，查询其遍历次数

mysql> drop index ind_sd1 on student;
Query OK, 0 rows affected (0.32 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> EXPLAIN select * from student where name='asd';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | student | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |    12.50 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

可以看出取消索引后，遍历次数为8.

因此加入索引可以大大提高数据库的检索效率！！！

创建索引的代码为：

CREATE INDEX 索引名 ON 表名(列名);

查看索引代码：

SHOW INDEX FROM 表名;

删除索引的代码：

DROP INDEX 索引名 ON 表名;

 

创建索引报错问题：

对以上的student表格创建唯一索引会报错

mysql> create unique index ind_sd1 on student(name);
ERROR 1062 (23000): Duplicate entry 'alex' for key 'ind_sd1'

试试创建双索引：

mysql> create index ind_sd1 on student(name,age);
Query OK, 0 rows affected (0.62 sec)
Records: 0  Duplicates: 0  Warnings: 0

当创建了唯一性索引，插入和该列相同的记录，系统会报错。 
如果已有数据重复，创建唯一性索引也会报错 
但是，如果创建了一个复合的唯一性索引，则只要两列不完全相同，数据可以成功插入