##---------mysql学习(四)索引的建立--------###

#今天突然开窍了，所以补充点索引方面的知识。

#创建索引，这里仍然以数据较少的mytab表为例：

#原数据为：

mysql> set names gbk;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from mytab;

+----+--------+-----+--------+

| id | name   | age | salary |

+----+--------+-----+--------+

|  1 | ?阿琼      |  23 |   1000 |

|  2 | 秋水虾     |  24 |    500 |

|  3 | 害人精      |  22 |    100 |

+----+--------+-----+--------+

3 rows in set (0.00 sec)

#alter  table table_name add index index_name (column)==

#create index index_name on table_name(column);

#alter创建索引示例

mysql> alter table mytab add index mytab_name (name);

Query OK, 3 rows affected (0.15 sec)

Records: 3  Duplicates: 0  Warnings: 0

#create创建索引示例：

mysql> create index mytab_id on mytab (id);

Query OK, 3 rows affected (0.16 sec)

Records: 3  Duplicates: 0  Warnings: 0

#查看索引

mysql> show index from mytab;

+-------+------------+----------+--------------+-------------+-----------+------

-------+----------+--------+------+------------+---------+

| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardi

nality | Sub_part | Packed | Null | Index_type | Comment |

+-------+------------+----------+--------------+-------------+-----------+------

-------+----------+--------+------+------------+---------+

| mytab |          0 | PRIMARY  |            1 | id          | A         |

3 |     NULL | NULL   |      | BTREE      |         |

| mytab |          1 | mytab_id |            1 | id          | A         |

3 |     NULL | NULL   |      | BTREE      |         |

+-------+------------+----------+--------------+-------------+-----------+------

-------+----------+--------+------+------------+---------+

2 rows in set (0.00 sec)

#创建unique索引

mysql> alter table mytab add unique (name);

Query OK, 3 rows affected (0.20 sec)

Records: 3  Duplicates: 0  Warnings: 0

#创建联合索引：

mysql> create index mytab_id_name on mytab (id,name);

Query OK, 3 rows affected (0.20 sec)

Records: 3  Duplicates: 0  Warnings: 0

mysql> show index from mytab;

+-------+------------+---------------+--------------+-------------+-----------+-

------------+----------+--------+------+------------+---------+

| Table | Non_unique | Key_name      | Seq_in_index | Column_name | Collation |

Cardinality | Sub_part | Packed | Null | Index_type | Comment |

+-------+------------+---------------+--------------+-------------+-----------+-

------------+----------+--------+------+------------+---------+

| mytab |          0 | PRIMARY       |            1 | id          | A         |

3 |     NULL | NULL   |      | BTREE      |         |

| mytab |          0 | name          |            1 | name        | A         |

3 |     NULL | NULL   |      | BTREE      |         |

| mytab |          1 | mytab_name    |            1 | name        | A         |

3 |     NULL | NULL   |      | BTREE      |         |

| mytab |          1 | mytab_id_name |            1 | id          | A         |

3 |     NULL | NULL   |      | BTREE      |         |

| mytab |          1 | mytab_id_name |            2 | name        | A         |

3 |     NULL | NULL   |      | BTREE      |         |

+-------+------------+---------------+--------------+-------------+-----------+-

------------+----------+--------+------+------------+---------+

5 rows in set (0.00 sec)

#下面我们尝试一下删除索引，删除用drop

#drop index index_name on table_name==

#alter table table_name drop index index_name;

#drop示例：

mysql> drop index mytab_id on mytab;

Query OK, 3 rows affected (0.17 sec)

Records: 3  Duplicates: 0  Warnings: 0

#alter示例：

mysql> alter table mytab drop index mytab_id_name;

Query OK, 3 rows affected (0.17 sec)

Records: 3  Duplicates: 0  Warnings: 0

#现在发现由于数据数量较小，根本无法判断索引存在的价值。

#

#这里我打算向其中添加3000行数据，这里需要用到Java代码：

#

| 3001 | yiha_2997 |  22 |   5997 |

| 3002 | yiha_2998 |  22 |   5998 |

| 3003 | yiha_2999 |  22 |   5999 |

+------+-----------+-----+--------+

3003 rows in set (0.01 sec)

#######################java代码段##############################

public static void main(String[] args) {

Connection conn=DBConnection.getConnection();

try {

conn.setAutoCommit(false);

PreparedStatement state=conn.prepareStatement

("insert into mytab(name,age,salary) values (?,?,?)");

for(int i=0;i<3000;i++){

state.setString(1,"yiha_"+i );

state.setInt(2, 22);

state.setInt(3, 3000+i);

state.addBatch();

}

state.executeBatch();

conn.commit();

state.close();

} catch (SQLException e) {

e.printStackTrace();

}

}

######################数据库连接connection######################

private static String url="jdbc:mysql://" +

"localhost:3306/mydb?useUnicode=true&characterEncoding=UTF-8";

private static String driver="com.mysql.jdbc.Driver";

private static String name="root";

private static String pwd="root";

public static Connection getConnection(){

Connection conn;

try {

Class.forName(driver).newInstance();

conn = DriverManager.getConnection(url, name, pwd);

return conn;

###################################################################

##现在数据库中有3003条数据，我们看一下检索数据时间。

#如检索：

id      NAME      age     salary

| 2894 | yiha_2890 |  22 |   5890 |

#id以及name为索引，但是age和salary为非索引

mysql> select * from mytab where id=2894;

+------+-----------+-----+--------+

| id   | name      | age | salary |

+------+-----------+-----+--------+

| 2894 | yiha_2890 |  22 |   5890 |

+------+-----------+-----+--------+

1 row in set (0.00 sec)

mysql> select * from mytab where salary=5890;

+------+-----------+-----+--------+

| id   | name      | age | salary |

+------+-----------+-----+--------+

| 2894 | yiha_2890 |  22 |   5890 |

+------+-----------+-----+--------+

1 row in set (0.00 sec)

#可以看出无差别，也许数据仍旧太少，现在将数据提升到30000；

mysql> select * from mytab where id=30000; #id为索引

+-------+------------+-----+--------+

| id    | name       | age | salary |

+-------+------------+-----+--------+

| 30000 | yiha_29996 |  23 |  32996 |

+-------+------------+-----+--------+

1 row in set (0.00 sec)

mysql> select * from mytab where salary=32996;#salary为非索引

+-------+------------+-----+--------+

| id    | name       | age | salary |

+-------+------------+-----+--------+

| 30000 | yiha_29996 |  23 |  32996 |

+-------+------------+-----+--------+

1 row in set (0.02 sec)

#由于name也是索引，所以这里试一下用name查找数据：

mysql> select * from mytab where name=‘yiha_29996‘;#name为索引

+-------+------------+-----+--------+

| id    | name       | age | salary |

+-------+------------+-----+--------+

| 30000 | yiha_29996 |  23 |  32996 |

+-------+------------+-----+--------+

1 row in set (0.00 sec)

##虽然在数据多次实验中能够看出索引的作用，但是并不是很明显。以上每一组所耗费时间都是

#个人寻找的出现次数最多的时间。

##个人感觉测试索引效果挺无聊的，索引的作用很多文章都只写了可以精确查找，至于索引如何

#运用貌似很少有相关的东西。数据库中的数据还可以随意扩大，个人感觉先这样吧。

原文：http://blog.csdn.net/trsli/article/details/19487949