Mysql運維管理-創建索引知識及創建索引的多種方法實戰9
為表的字段創建索引
索引就像書的目錄一樣,如果在字段上建立索引,那麽以索引為條件時可以加快查詢數據的速度。
1 創建主鍵索引
查詢數據庫的內容,按主鍵查詢是最快的,每個表只能有一個主鍵,但是可以有多個普通索引列,主鍵列要求所有內容必須唯一,而索引列不要求內容唯一。
我們無論建立主鍵索引還是普通索引,都要在表的對應列上創建,可以對單列創建索引也可以對多列創建索引
建立主鍵索方法:
1.在創建表時,可以增加建立主鍵索引語句
system@ceshi 04:0932->create table student( -> id int(4) not null AUTO_INCREMENT, -> name char(20) not null, -> age tinyint(2) NOT NULL default ‘0‘, -> dept varchar(16) default NULL, -> primary key(id), -> KEY index_name(name) -> );
提示:
a.AUTO_INCREMENT 自增
b.Primary key(id) 主鍵
c.KEY index_name(name) name 字段普通索引
操作演示
system@ceshi 04:2146->drop table student; Query OK, 0 rows affected (0.00 sec) system@ceshi 04:2333->create table student( -> id int(4) not null AUTO_INCREMENT, -> name char(20) not null, -> age tinyint(2) NOT NULL default ‘0‘, -> dept varchar(16) default NULL, -> primary key(id), -> KEY index_name(name) -> ); Query OK, 0 rows affected (0.00 sec) system@ceshi 04:2609->desc student; +-------+-------------+------+-----+---------+----------------+ | Field | Type| Null | Key | Default | Extra | +-------+-------------+------+-----+---------+----------------+ | id| int(4) | NO | PRI | NULL| auto_increment | | name | char(20)| NO | MUL | NULL|| | age | tinyint(2) | NO | | 0 || | dept | varchar(16) | YES | | NULL|| +-------+-------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)
提示:PRI為主鍵的標識,MUL為普通索引的標識。
2.建立表之後通過alter命令增加主鍵索引(不推薦這種做法)
a.主鍵列不能重復創建,必須先刪除上面的配置
system@ceshi 04:3716->alter table student drop primary key;
b.建表時忘記加主鍵了利用alter命令增加id列為自增主鍵列
system@ceshi 04:5948->alter table student change id id int primary key auto_increment; Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0 system@ceshi 05:0324->desc student; +-------+-------------+------+-----+---------+----------------+ | Field | Type| Null | Key | Default | Extra | +-------+-------------+------+-----+---------+----------------+ | id| int(11) | NO | PRI | NULL| auto_increment | | name | char(20)| NO | MUL | NULL|| | age | tinyint(2) | NO | | 0 || | dept | varchar(16) | YES | | NULL|| +-------+-------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)
提示:只有int類型且為primary key才可以使用auto_increment。
9.9.7.2 創建普通索引
1.在建表時,可以增加建立普通索引列的語句如下:
system@ceshi 04:0932->create table student(
-> id int(4) not null AUTO_INCREMENT,
-> name char(20) not null,
-> age tinyint(2) NOT NULL default ‘0‘,
-> dept varchar(16) default NULL,
-> primary key(id),
-> KEY index_name(name)
-> );
提示:
a.KEY index_name(name)name 字段普通索引
操作演示:
system@ceshi 04:2146->drop table student;
Query OK, 0 rows affected (0.00 sec)
system@ceshi 04:2333->create table student(
-> id int(4) not null AUTO_INCREMENT,
-> name char(20) not null,
-> age tinyint(2) NOT NULL default ‘0‘,
-> dept varchar(16) default NULL,
-> primary key(id),
-> KEY index_name(name)
-> );
Query OK, 0 rows affected (0.00 sec)
system@ceshi 04:2609->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(4) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
2.建表後利用alter增加普通索引
刪除建表時創建的index_name索引
system@ceshi 05:0333->alter table student drop index index_name;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
system@ceshi 06:0419->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
在name列上添加索引,索引名為index_name
system@ceshi 06:0431->alter table student add index index_name(name);
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
system@ceshi 06:0456->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
3 對字段的前n個字符創建普通索引
當遇到表中比較大的列時,列內容的前n個字符在所有內容中已經接近唯一時,這時可以對列的前n個字符建立索引而無需對整個列建立索引,這樣可以節省創建索引占用的系統空間,以及降低和更新維護消耗的系統資源。
對字段的前n個字符創建索引的語法:
Create index index_name on test(name(8)); 條件列前n個字符創建索引
實戰操作:
system@ceshi 06:0838->create index index_dept on student(dept(8));
Query OK, 0 rows affected (0.19 sec)
Records: 0 Duplicates: 0 Warnings: 0
system@ceshi 06:3650->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | MUL | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
system@ceshi 06:3847->show index from student\G
*************************** 1. row ***************************
Table: student
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 2. row ***************************
Table: student
Non_unique: 1
Key_name: index_name
Seq_in_index: 1
Column_name: name
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 3. row ***************************
Table: student
Non_unique: 1
Key_name: index_dept
Seq_in_index: 1
Column_name: dept
Collation: A
Cardinality: NULL
Sub_part: 8
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
3 rows in set (0.00 sec)
4 為表的多個字段創建聯合索引
如果查詢的條件是多列時,我們可以為多個查詢的列創建聯合索引,甚至可以為多列的前n個字符創建聯合索引,實戰演示如下:
為多個列創建索引
system@ceshi 06:4019->create index ind_name_dept on student(name,dept);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
system@ceshi 06:4726->show index from student\G
*************************** 1. row ***************************
Table: student
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 2. row ***************************
Table: student
Non_unique: 1
Key_name: index_name
Seq_in_index: 1
Column_name: name
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 3. row ***************************
Table: student
Non_unique: 1
Key_name: index_dept
Seq_in_index: 1
Column_name: dept
Collation: A
Cardinality: NULL
Sub_part: 8
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
*************************** 4. row ***************************
Table: student
Non_unique: 1
Key_name: ind_name_dept
Seq_in_index: 1
Column_name: name
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 5. row ***************************
Table: student
Non_unique: 1
Key_name: ind_name_dept
Seq_in_index: 2
Column_name: dept
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
5 rows in set (0.00 sec)
可以對多個列的前n個字符創建聯合索引,實戰演示如下:
system@ceshi 06:5144->drop index ind_name_dept on student; 刪除索引的另一種方法。
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
system@ceshi 06:5158->create index ind_name_dept on student(name(8),dept(10));
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
提示:
按條件列查詢數據時,聯合索引是有前綴生效特性的
Index(a,b,c)僅a,ab,abc三個查詢條件可以走索引。b,bc,ac,c不能走索引
5 創建唯一索引(非主鍵索引)
Create unique index index_age on student(age);
唯一索引是用來約束表的內容的,不能重復。
system@ceshi 07:0828->create unique index uni_ind_name on student(name);
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
system@ceshi 07:1010->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | UNI | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | MUL | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
6 索引表的創建及生效條件
問題1:既然索引可以加快查詢速度,那麽是不是就要給所有的列創建索引呢?
解答:因為索引你不但占用系統空間,更新數據庫時還需要維護索引數據。因此索引是一把雙刃劍,並不是越多越好,例如:數十到幾百行的小表上無需創建索引,更新頻繁,讀取少的業務要少建立索引。
問題2:需要在哪些列上創建索引呢?
解答:Select user,host from mysql.user where host= ….索引一定要創建在where後的條件列上,而不是select後的選擇數據的列上,另外我們要盡量選擇在唯一值多的大表上建立索引。創建索引要和開發商量。
7 創建索引命令集合小結
1.創建索引相關命令集合
創建主鍵索引
alter table student change id id int primary key auto_increment;
刪除主鍵索引
alter table student drop primary key
創建普通索引
alter table student add index index_dept(dept(8))
根據列的前n個字符創建索引
create index index_dept on student(dept(8))
根據多個列創建聯合索引
create index ind_name_dept on student(name,dept);
根據多個列的前n個字符創建索引
create index ind_name_dept on student(name(8),dept(10));
刪除普通索引
Alter table student drop index index_dept
drop index ind_name_dept on student
創建唯一索引
create unique index uni_ind_name on student(name);
基本索引:
a.要在表的列上創建索引。
b.索引會加快查詢速度,但是會影響更新的速度,因為要維護索引。
c.索引不是越多越好,要在頻繁查詢的where後的條件列上創建索引。
d.小表或唯一值極少的列上不建索引,要在大表以及不同內容多的列上創建索引。
Mysql運維管理-創建索引知識及創建索引的多種方法實戰9