index 普通索引

3.1 问题

具体要求如下：

在已有表里添加index字段
建表时，添加index字段
查看表索引
删除表索引

3.2 步骤

实现此案例需要按照如下步骤进行。

步骤一：索引的创建与删除

1）创建表的时候指定INDEX索引字段

创建库home：

mysql> create database home;
Query OK, 1 row affected (0.00 sec)

允许有多个INDEX索引字段。比如，以下操作在home库中创建了tea4表，将其中的id、name作为索引字段：

mysql> USE home;
Database changed
mysql> CREATE TABLE tea4(
    -> id char(6) NOT NULL,
    -> name varchar(6) NOT NULL,
    -> age int(3) NOT NULL,
    -> gender ENUM('boy','girl') DEFAULT 'boy',
    -> INDEX(id),INDEX(name)
    -> );
Query OK, 0 rows affected (0.59 sec)

查看新建tea4表的字段结构，可以发现两个非空索引字段的KEY标志为MUL：

mysql> DESC tea4;
+--------+--------------------+------+-----+---------+-------+
| Field  | Type               | Null | Key | Default | Extra |
+--------+--------------------+------+-----+---------+-------+
| id     | char(6)            | NO   | MUL | NULL    |       |
| name   | varchar(6)         | NO   | MUL | NULL    |       |
| age    | int(3)             | NO   |     | NULL    |       |
| gender | enum('boy','girl') | YES  |     | boy     |       |
+--------+--------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

2）删除现有表的某个INDEX索引字段

比如，删除tea4表中名称为named的INDEX索引字段：

mysql> drop INDEX name ON tea4;                  //删除name字段的索引
Query OK, 0 rows affected (0.18 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> DESC tea4;                                      //确认删除结果
+--------+--------------------+------+-----+---------+-------+
| Field  | Type               | Null | Key | Default | Extra |
+--------+--------------------+------+-----+---------+-------+
| id     | char(6)            | NO   | MUL | NULL    |       |
| name   | varchar(6)         | NO   |     | NULL    |       |
| age    | int(3)             | NO   |     | NULL    |       |
| gender | enum('boy','girl') | YES  |     | boy     |       |
+--------+--------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

3）在已有的某个表中设置INDEX索引字段

比如，针对tea4表的age字段建立索引，名称为 nianling：

mysql> CREATE INDEX nianling ON tea4(age);      //针对指定字段创建索引
Query OK, 0 rows affected (0.62 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> DESC tea4;                                  //确认创建结果
+--------+--------------------+------+-----+---------+-------+
| Field  | Type               | Null | Key | Default | Extra |
+--------+--------------------+------+-----+---------+-------+
| id     | char(6)            | NO   | MUL | NULL    |       |
| name   | varchar(6)         | NO   |     | NULL    |       |
| age    | int(3)             | NO   | MUL | NULL    |       |
| gender | enum('boy','girl') | YES  |     | boy     |       |
+--------+--------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

4）查看指定表的索引信息

使用SHOW INDEX 指令：

mysql> SHOW INDEX FROM tea4\G
*************************** 1. row ***************************
        Table: tea4
   Non_unique: 1
     Key_name: id
 Seq_in_index: 1
  Column_name: id
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: 
   Index_type: BTREE                          //使用B树算法
      Comment: 
Index_comment: 
*************************** 2. row ***************************
        Table: tea4
   Non_unique: 1
     Key_name: nianling                       //索引名称
 Seq_in_index: 1
  Column_name: age                            //字段名称
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: 
   Index_type: BTREE
      Comment: 
Index_comment: 
2 rows in set (0.00 sec)
Query OK, 0 rows affected (0.30 sec)
Mysql>
Query OK, 0 rows affected (0.47 sec)
Records: 0  Duplicates: 0  Warnings: 0