1.什么是索引
索引是一種特殊的文件(InnoDB數據表上的索引是表空間的一個組成部分)，它們包含着對數據表里所有記錄的引用指針。
更通俗的說，數據庫索引好比是一本書前面的目錄，在查找內容之前可以先在目錄中查找索引位置，以此快速定位查詢數據。
對於索引，會保存在額外的文件中。
2.索引的類型
1、普通索引
2、唯一性索引
3、主鍵索引（主索引）
4、復合索引
5、全文索引
2.1. 普通索引
最基本的索引，不具備唯一性，僅加快查詢速度
2.1.1 創建普通索引
方法一：創建表時添加索引
語法：
create table 表名（
列定義
index 索引名稱 （字段）
index 索引名稱 （字段）
）
注：可以使用key關鍵字，也可以使用index關鍵字。索引名稱，可以加也可以不加，不加使用字段名作為索引名。
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
MariaDB [book]> create table demo(id int(4),name varchar(20),pwd varchar(20),index(pwd));
Query OK, 0 rows affected (0.08 sec)
MariaDB [book]> create table demo1(id int(4),name varchar(20),pwd varchar(20),key(pwd));
Query OK, 0 rows affected (0.01 sec)
MariaDB [book]> create table demo2(id int(4),name varchar(20),pwd varchar(20),key index_pwd(pwd)); # 為索引加上名稱
Query OK, 0 rows affected (0.02 sec)
注意
index和 key作用是相同的
方法二：當表創建完成后，使用alter為表添加索引：
語法：
alter table 表名 add index 索引名稱 （字段1，字段2.....）;
例子：
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
MariaDB [book]> alter table books add key (bName,bTypeId);
Query OK, 44 rows affected (0.01 sec)
Records: 44 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books; # 可以看到在bName字段上添加的索引
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
2.1.2 查看索引
語法：
desc 索引名(或索引字段)
例子：
MariaDB [book]> desc demo;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| pwd | varchar(20) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
MariaDB [book]> desc demo1;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| pwd | varchar(20) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
MariaDB [book]> desc demo2;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| pwd | varchar(20) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
注意：如果Key是MUL, 那么該列的值可以重復。該列是一個非唯一索引的前導列(第一列)或者是一個唯一性索引的組成部分但是可以含有空值NULL。
2.1.3 刪除索引
語法：
alter table 表名 drop key 索引名(或字段名);
例子：
MariaDB [book]> alter table demo drop key pwd;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [book]> alter table demo1 drop key pwd;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [book]> alter table demo2 drop key index_pwd;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [book]> alter table books add key (bName,bTypeId);
Query OK, 44 rows affected (0.01 sec)
Records: 44 Duplicates: 0 Warnings: 0
2.2 唯一索引
與普通索引基本相同，但有一個區別：
索引列的所有值都只能出現一次，即必須唯一，用來約束內容。
字段值只能出現一次時，應該加唯一索引。
唯一性允許有NULL值<允許為空>。
2.2.1 創建唯一索引
方法一：創建表時加唯一索引
語法：
create table 表名(
列定義：
unique key 索引名 （字段）;
)
注意：常用在值不能重復的字段上，比如說用戶名，電話號碼，身份證號。
例子：
MariaDB [book]> create table demo3(id int(4) auto_increment primary key,uName varchar(20),uPwd varchar(20),unique index(uName));
Query OK, 0 rows affected (0.07 sec)
MariaDB [book]> desc demo3;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| uName | varchar(20) | YES | UNI | NULL | |
| uPwd | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
方法二：修改表時加唯一索引
語法：
alter table 表名 add unique 索引名 (字段);
例子：
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.01 sec)
MariaDB [book]> alter table books add unique index_bName (bName);
Query OK, 43 rows affected (0.01 sec)
Records: 43 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | UNI | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.02 sec)
2.2.2 刪除唯一索引
alter table 表名 drop key 索引名(或字段名);
例子：
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.01 sec)
Records: 43 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.01 sec)
MariaDB [book]> desc demo3;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| uName | varchar(20) | YES | UNI | NULL | |
| uPwd | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
MariaDB [book]> alter table demo3 drop key uName;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [book]> desc demo3;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| uName | varchar(20) | YES | | NULL | |
| uPwd | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
2.3 主鍵索引
查詢數據庫，按主鍵查詢是最快的，每個表只能有一個主鍵列，可以有多個普通索引列。
主鍵列要求列的所有內容必須唯一，而索引列不要求內容必須唯一，但不允許為空
2.3.1 創建主鍵索引
方法一：創建表創建主鍵索引
MariaDB [book]> create table demo4 (id int(4) not null auto_increment,name varchar(4) default null,primary key(id));
Query OK, 0 rows affected (0.02 sec)
MariaDB [book]> desc demo4;
+-------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| name | varchar(4) | YES | | NULL | |
+-------+------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)
MariaDB [book]> show index from demo4;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| demo4 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
方法二：創建表后添加<不推薦>
MariaDB [book]> drop table demo4;
Query OK, 0 rows affected (0.01 sec)
MariaDB [book]> create table demo4 (id int(4) not null auto_increment primary key,name varchar(4) not null);
Query OK, 0 rows affected (0.02 sec)
MariaDB [book]> alter table demo4 change id id int(4) not null;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [book]> desc demo4;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(4) | NO | PRI | NULL | |
| name | varchar(4) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
MariaDB [book]> show index from demo4;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| demo4 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
2.3.2 刪除主鍵索引
MariaDB [book]> alter table demo4 drop primary key;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [book]> desc demo4;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(4) | NO | | NULL | |
| name | varchar(4) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
MariaDB [book]> show index from demo4;
Empty set (0.00 sec)
主鍵索引，唯一性索引區別：主鍵索引不能有NULL，唯一性索引可以有空值
2.4 復合索引
索引可以包含一個、兩個或更多個列。兩個或更多個列上的索引被稱作復合索引
復合索引專門用於組合搜索，其效率大於索引合並
例： 創建一個表存放服務器允許或拒絕的IP和port，但記錄中IP和port要唯一。
MariaDB [book]> create table firewall(host varchar(15) not null,port smallint(4) not null,access enum("deny","allow") not null,primary key(host,port)); # 用host和port列作復合索引
Query OK, 0 rows affected (0.11 sec)
MariaDB [book]> insert into firewall values("192.168.81.100",22,"deny");
Query OK, 1 row affected (0.02 sec)
MariaDB [book]> select * from firewall;
+---------------+------+--------+
| host | port | access |
+---------------+------+--------+
| 192.168.81.100 | 22 | deny |
+---------------+------+--------+
1 row in set (0.00 sec)
MariaDB [book]> insert into firewall values("192.168.81.101",22,"allow");
Query OK, 1 row affected (0.04 sec)
MariaDB [book]> insert into firewall values("192.168.81.100",22,"deny"); # 插入數據時，IP和port要唯一，否則會報錯
ERROR 1062 (23000): Duplicate entry '192.168.81.100-22' for key 'PRIMARY'
MariaDB [book]> select * from firewall;
+----------------+------+--------+
| host | port | access |
+----------------+------+--------+
| 192.168.81.100 | 22 | deny |
| 192.168.81.101 | 22 | allow |
+----------------+------+--------+
2 rows in set (0.01 sec)
MariaDB [book]> desc firewall; # 查看索引
+--------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+----------------------+------+-----+---------+-------+
| host | varchar(15) | NO | PRI | NULL | |
| port | smallint(4) | NO | PRI | NULL | |
| access | enum('deny','allow') | NO | | NULL | |
+--------+----------------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
建表的時候如果加各種索引，順序如下：
create table 表名（字段定義，PRIMARYKEY (`bId`),UNIQUE KEY `bi` (`bImg`),KEY `bn` (`bName`),KEY `ba` (`author`)）
2.5 全文索引 (FULLTEXT INDEX)
全文索引(也稱全文檢索)是目前搜索引擎使用的一種關鍵技術。
它能夠利用"分詞技術"等多種算法智能分析出文本文字中關鍵字詞的頻率及重要性，然后按照一定的算法規則智能地篩選出我們想要的搜索結果
mysql 在數據量較大的情況下，高並發連接的情況下:
select 語句 where bName like '%網%'
使用% 和 _ 通配符進行查找，因為索引里面保存的是一堆指針，不會保存具體內容，只能進行全表掃描，造成數據庫壓力大
全文索引只能用在varchar和text字段中
2.5.1 創建全文索引
方法一：創建表時創建
語法：
create table 表名（
列定義，
fulltext key 索引名 （字段）;
）
方法二：修改表時添加
語法：
alter table 表名 add fulltext 索引名 （字段）;
例子：
MariaDB [book]> show create table books;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| books | CREATE TABLE `books` (
`bId` int(4) NOT NULL AUTO_INCREMENT,
`bName` varchar(255) DEFAULT NULL,
`bTypeId` enum('1','2','3','4','5','6','7','8','9','10') DEFAULT NULL,
`publishing` varchar(255) DEFAULT NULL,
`price` int(4) DEFAULT NULL,
`pubDate` date DEFAULT NULL,
`author` varchar(30) DEFAULT NULL,
`ISBN` varchar(255) DEFAULT NULL,
PRIMARY KEY (`bId`)
) ENGINE=MyISAM AUTO_INCREMENT=45 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
MariaDB [book]> alter table books add fulltext index_bName (`bName`);
Query OK, 43 rows affected (0.04 sec)
Records: 43 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.03 sec)
Records: 43 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
MariaDB [book]> alter table books add fulltext index_bName (`bName`,`publishing`);
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.01 sec)
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
MariaDB [book]> alter table books add fulltext index_bName (`publishing`);
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | MUL | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+
8 rows in set (0.01 sec)
2.5.2 刪除全文索引
MariaDB [book]> alter table books drop key index_bName;
Query OK, 39 rows affected (0.02 sec)
Records: 39 Duplicates: 0 Warnings: 0
MySQL自帶的全文索引只能用於數據庫引擎為MyISAM的數據表，如果是其他數據引擎，則全文索引不會生效
MySQL自帶的全文索引只能對英文進行全文檢索，目前無法對中文進行全文檢索。
3.索引設計原則
1、索引並非越多越好
2、數據量不大不需要建立索引
3、列中的值變化不多不需要建立索引 row id
4、經常排序（order by）和分組（group by）的列需要建立索引
5、唯一性約束對應使用唯一性索引
4.索引的優點：
由於索引是專門用於加速搜索而生，所以加上索引之后，查詢效率會很快，查詢時間會減少
5.索引的缺點
1.索引是以文件存儲的。如果索引過多，占磁盤空間較大。而且他影響： insert ,update ,delete 執行時間。
2.索引中數據必須與數據表數據同步：如果索引過多，當表中數據更新的時候后，索引也要同步更新，這就降低了效率。