##---------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=DBConn