##---------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;
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##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。
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