标签：prim inno group record csharp 分组 情况 允许 cte
索引是一种特殊的文件，包含了对数据表中所有记录的引用指针。InnoDB引擎的数据库，其上的索引是表空间的一个组成部分。
(1).索引的优缺点
优点：加快搜索速度，减少查询时间
缺点：索引是以文件的形式存储，如果索引过多，会占用磁盘较大的空间。而且影响insert、update、delete的执行时间。
索引中的数据必须与数据表中的人数据同步，如果索引过多，当表中数据更新，索引也要同步更新，这就降低了效率。
(2).普通索引
最基本的索引，不具备唯一性，仅加快查询速度。
1）创建普通索引
create table [表名] ([字段名] [字段类型] [字段约束],...,[ index | key ] [索引名称]([字段名称]));
mysql> create table com_index1(id int,name varchar(20),key (name)); Query OK, 0 rows affected (0.02 sec) mysql> desc com_index1; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | MUL | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> create table com_index2(id int,name varchar(20),index com_name(name)); Query OK, 0 rows affected (0.11 sec) mysql> desc com_index2; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | MUL | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.03 sec) mysql> create table com_index(id int,name varchar(20),index (name)); Query OK, 0 rows affected (0.02 sec) mysql> desc com_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | MUL | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
注意1：索引文件是一表一个，所以在没有外键约束的情况下，只要表内没有重名索引即可。
注意2：在Key列值为MUL的一般是普通索引，可以使用show index from [表名]来确认索引类型，里面有索引类型列如果为BTREE那么就是普通索引。
注意3：如果没有指定索引名，那么默认索引名与字段名一致。
2）添加普通索引
alter table [表名] add [ index | key ] [索引名称]([字段名]);
mysql> alter table com_index1 add key (id); Query OK, 0 rows affected (0.11 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc com_index1; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | MUL | NULL | | | name | varchar(20) | YES | MUL | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> alter table com_index2 add index com_id(id); Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc com_index2; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | MUL | NULL | | | name | varchar(20) | YES | MUL | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> alter table com_index add index (id); Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc com_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | MUL | NULL | | | name | varchar(20) | YES | MUL | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
3）删除普通索引
alter table [表名] drop [ key | index ] [索引名];
mysql> alter table com_index1 drop key name; Query OK, 0 rows affected (0.20 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc com_index1; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | MUL | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> alter table com_index1 drop index id; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc com_index1; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.01 sec)
(3).唯一索引
设置唯一索引的列中所有值都只能出现一次，允许NULL。唯一索引一般只在创建数据表以及数据表没有数据之前创建或添加，如果已经存在数据，必须保证添加的列所有值具有唯一性。
1）创建唯一索引
create table [表名] ([字段名] [字段类型] [字段约束],...,unique [索引名称]([字段名称]));
mysql> create table uni_index(id int,name varchar(20),unique (id)); Query OK, 0 rows affected (0.12 sec) mysql> desc uni_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | UNI | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
注意：在Key列值为UNI的即是唯一索引。
2）添加唯一索引（尽量不要用，用也尽量在存放数据之前）
alter table [表名] add unique [索引名]([字段名]);
mysql> alter table uni_index add unique (name); Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc uni_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | UNI | NULL | | | name | varchar(20) | YES | UNI | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
3）删除唯一索引
alter table [表名] drop key [索引名];
mysql> alter table uni_index drop key name; Query OK, 0 rows affected (0.13 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc uni_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | UNI | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
(4).主键索引
查询数据库按主键查询是最快的，每个数据表只允许存在一个主键，主键必须唯一，不允许为NULL。主键和唯一索引一样，一般只在创建数据表以及数据表没有数据之前创建或添加，如果已经存在数据，必须保证添加的列所有值具有唯一性。
1）创建主键
create table [表名] ([字段名] [字段类型] [字段约束],...,primary key [索引名称]([字段名称]));
create table [表名] ([字段名] [字段类型] [字段约束] [ primary key | key ],[字段名] [字段类型] [字段约束]...);
mysql> create table pri_index1 (id int,primary key (id)); Query OK, 0 rows affected (0.01 sec) mysql> desc pri_index1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.01 sec) mysql> create table pri_index2 (id int key); Query OK, 0 rows affected (0.01 sec) mysql> desc pri_index2; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec) mysql> create table pri_index3 (id int primary key); Query OK, 0 rows affected (0.02 sec) mysql> desc pri_index3; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec)
注意：在Key列值为PRI的即是主键索引。
2）添加主键（尽量不要用，用也尽量在存放数据之前）
alter table [表名] add primary key [索引名]([字段名]);
mysql> create table pri_index4 (id int); Query OK, 0 rows affected (0.02 sec) mysql> desc pri_index4; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec) mysql> alter table pri_index4 add primary key (id); Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc pri_index4; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec)
3）删除主键
alter table [表名] drop primary key;
mysql> alter table pri_index4 drop primary key; Query OK, 0 rows affected (0.13 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc pri_index4; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.01 sec)
(5).复合索引
一个索引可以包含一个或一个以上的列，当索引包含两个及以上的列时，此时的索引被称之为复合索引。基本创建、添加、删除方式没有改变，只是字段名部分可以为一个以上，之间由逗号隔开。
创建
mysql> create table comp_index(id int,name varchar(20), primary key (id,name)); Query OK, 0 rows affected (0.01 sec) mysql> desc comp_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | NO | PRI | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
添加
mysql> create table comp_index1(id int,name varchar(20)); Query OK, 0 rows affected (0.25 sec) mysql> alter table comp_index1 add primary key (id,name); Query OK, 0 rows affected (0.17 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc comp_index1; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | NO | PRI | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
删除
mysql> alter table comp_index drop primary key; Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc comp_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | | name | varchar(20) | NO | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
(6).全文索引
全文索引也称全文检索，是目前搜索引擎使用的一种关键技术。它能够利用多种算法智能分析出文本文字中关键词的频率和重要性，然后按照一定的算法规则智能筛选出想要的结果。3.2开始支持全文索引，但无法正确支持中文；5.7.6开始内置ngram全文检索插件，用来支持中文。
旧版的全文索引只能用在MyISAM数据库引擎的表上，但5.6.24上InnoDB也加入了全文索引。不够只支持char、varchar和text的字段类型。
1）创建全文索引
create table [表名] ([字段名] [字段类型] [字段约束],...,fulltext key [索引名称]([字段名称]));
mysql> create table ful_index(id int,name varchar(20),age varchar(20),fulltext key (name)); Query OK, 0 rows affected (0.25 sec) mysql> desc ful_index; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | MUL | NULL | | | age | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 3 rows in set (0.00 sec) mysql> show index from ful_index; +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | ful_index | 1 | name | 1 | name | NULL | 0 | NULL | NULL | YES | FULLTEXT | | | +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 1 row in set (0.00 sec)
注意：这里Key的值虽然是MUL，但使用show index from [表名]可以看到实际索引类型是FULLTEXT。
2）添加全文索引
alter table [表名] add fulltext key [索引名]([字段名]);
mysql> alter table ful_index add fulltext key (age); Query OK, 0 rows affected (0.26 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> show index from ful_index; +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | ful_index | 1 | name | 1 | name | NULL | 0 | NULL | NULL | YES | FULLTEXT | | | | ful_index | 1 | age | 1 | age | NULL | 0 | NULL | NULL | YES | FULLTEXT | | | +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 2 rows in set (0.01 sec)
3）删除全文索引
alter table [表名] drop key [索引名];
mysql> alter table ful_index drop key age; Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> show index from ful_index; +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | ful_index | 1 | name | 1 | name | NULL | 0 | NULL | NULL | YES | FULLTEXT | | | +-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 1 row in set (0.00 sec)
(7).索引创建原则
索引并不是越多越好：1.数据量不大时不需要创建索引；
2.列中值变化种类不多不需要创建索引；
3.经常排序（order by）和分组（group by）的列需要建立索引；
4.列中值需要具有唯一性，可以使用唯一索引或主键索引，但每个表中只能存在一个主键索引，并且不能为NULL。
标签：prim inno group record csharp 分组 情况 允许 cte
原文地址：https://www.cnblogs.com/diantong/p/10985718.html
