閱讀目錄

• 一 介紹
• 二 not null與default
• 三 unique
• 四 primary key
• 五 auto_increment
• 六 foreign key
• 七?總結(jié)

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一 介紹

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約束條件與數(shù)據(jù)類型的寬度一樣，都是可選參數(shù)

作用：用于保證數(shù)據(jù)的完整性和一致性
主要分為：

PRIMARY KEY (PK) 標(biāo)識該字段為該表的主鍵，可以唯一的標(biāo)識記錄 FOREIGN KEY (FK) 標(biāo)識該字段為該表的外鍵 NOT NULL 標(biāo)識該字段不能為空 UNIQUE KEY (UK) 標(biāo)識該字段的值是唯一的 AUTO_INCREMENT 標(biāo)識該字段的值自動增長（整數(shù)類型，而且為主鍵） DEFAULT 為該字段設(shè)置默認(rèn)值UNSIGNED 無符號 ZEROFILL 使用0填充

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說明：

1. 是否允許為空，默認(rèn)NULL，可設(shè)置NOT NULL，字段不允許為空，必須賦值 2. 字段是否有默認(rèn)值，缺省的默認(rèn)值是NULL，如果插入記錄時不給字段賦值，此字段使用默認(rèn)值 sex enum('male','female') not null default 'male' age int unsigned NOT NULL default 20 必須為正值（無符號） 不允許為空 默認(rèn)是20 3. 是否是key 主鍵 primary key 外鍵 foreign key 索引 (index,unique...)

二 not null與default

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是否可空，null表示空，非字符串
not null - 不可空
null - 可空

默認(rèn)值，創(chuàng)建列時可以指定默認(rèn)值，當(dāng)插入數(shù)據(jù)時如果未主動設(shè)置，則自動添加默認(rèn)值
create table tb1(
nid int not null defalut 2,
num int not null)

==================not null==================== mysql> create table t1(id int); #id字段默認(rèn)可以插入空 mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t1 values(); #可以插入空 mysql> create table t2(id int not null); #設(shè)置字段id不為空 mysql> desc t2; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t2 values(); #不能插入空 ERROR 1364 (HY000): Field 'id' doesn't have a default value==================default==================== #設(shè)置id字段有默認(rèn)值后，則無論id字段是null還是not null，都可以插入空，插入空默認(rèn)填入default指定的默認(rèn)值 mysql> create table t3(id int default 1); mysql> alter table t3 modify id int not null default 1;==================綜合練習(xí)==================== mysql> create table student(-> name varchar(20) not null,-> age int(3) unsigned not null default 18,-> sex enum('male','female') default 'male',-> hobby set('play','study','read','music') default 'play,music'-> ); mysql> desc student; +-------+------------------------------------+------+-----+------------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------------------------------+------+-----+------------+-------+ | name | varchar(20) | NO | | NULL | | | age | int(3) unsigned | NO | | 18 | | | sex | enum('male','female') | YES | | male | | | hobby | set('play','study','read','music') | YES | | play,music | | +-------+------------------------------------+------+-----+------------+-------+ mysql> insert into student(name) values('egon'); mysql> select * from student; +------+-----+------+------------+ | name | age | sex | hobby | +------+-----+------+------------+ | egon | 18 | male | play,music | +------+-----+------+------------+ 驗(yàn)證

三 unique

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============設(shè)置唯一約束 UNIQUE=============== 方法一： create table department1( id int, name varchar(20) unique, comment varchar(100) );方法二： create table department2( id int, name varchar(20), comment varchar(100), constraint uk_name unique(name) );mysql> insert into department1 values(1,'IT','技術(shù)'); Query OK, 1 row affected (0.00 sec) mysql> insert into department1 values(1,'IT','技術(shù)'); ERROR 1062 (23000): Duplicate entry 'IT' for key 'name' View Code mysql> create table t1(id int not null unique); Query OK, 0 rows affected (0.02 sec)mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ row in set (0.00 sec) not null+unique的化學(xué)反應(yīng) create table service( id int primary key auto_increment, name varchar(20), host varchar(15) not null, port int not null, unique(host,port) #聯(lián)合唯一 );mysql> insert into service values-> (1,'nginx','192.168.0.10',80),-> (2,'haproxy','192.168.0.20',80),-> (3,'mysql','192.168.0.30',3306)-> ; Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0mysql> insert into service(name,host,port) values('nginx','192.168.0.10',80); ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host' 聯(lián)合唯一

四 primary key

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primary key字段的值不為空且唯一

一個表中可以：

單列做主鍵
多列做主鍵（復(fù)合主鍵）

但一個表內(nèi)只能有一個主鍵primary key

============單列做主鍵=============== #方法一：not null+unique create table department1( id int not null unique, #主鍵 name varchar(20) not null unique, comment varchar(100) );mysql> desc department1; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | NO | UNI | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec)#方法二：在某一個字段后用primary key create table department2( id int primary key, #主鍵 name varchar(20), comment varchar(100) );mysql> desc department2; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.00 sec)#方法三：在所有字段后單獨(dú)定義primary key create table department3( id int, name varchar(20), comment varchar(100), constraint pk_name primary key(id); #創(chuàng)建主鍵并為其命名pk_name mysql> desc department3; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) 單列主鍵 ==================多列做主鍵================ create table service( ip varchar(15), port char(5), service_name varchar(10) not null, primary key(ip,port) );mysql> desc service; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | ip | varchar(15) | NO | PRI | NULL | | | port | char(5) | NO | PRI | NULL | | | service_name | varchar(10) | NO | | NULL | | +--------------+-------------+------+-----+---------+-------+ rows in set (0.00 sec)mysql> insert into service values-> ('172.16.45.10','3306','mysqld'),-> ('172.16.45.11','3306','mariadb')-> ; Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0mysql> insert into service values ('172.16.45.10','3306','nginx'); ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY' 多列主鍵

五 auto_increment

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約束字段為自動增長，被約束的字段必須同時被key約束

#不指定id，則自動增長 create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' );mysql> desc student; +-------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | YES | | male | | +-------+-----------------------+------+-----+---------+----------------+ mysql> insert into student(name) values-> ('egon'),-> ('alex')-> ;mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | | 2 | alex | male | +----+------+------+#也可以指定id mysql> insert into student values(4,'asb','female'); Query OK, 1 row affected (0.00 sec)mysql> insert into student values(7,'wsb','female'); Query OK, 1 row affected (0.00 sec)mysql> select * from student; +----+------+--------+ | id | name | sex | +----+------+--------+ | 1 | egon | male | | 2 | alex | male | | 4 | asb | female | | 7 | wsb | female | +----+------+--------+#對于自增的字段，在用delete刪除后，再插入值，該字段仍按照刪除前的位置繼續(xù)增長 mysql> delete from student; Query OK, 4 rows affected (0.00 sec)mysql> select * from student; Empty set (0.00 sec)mysql> insert into student(name) values('ysb'); mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 8 | ysb | male | +----+------+------+#應(yīng)該用truncate清空表，比起delete一條一條地刪除記錄，truncate是直接清空表，在刪除大表時用它 mysql> truncate student; Query OK, 0 rows affected (0.01 sec)mysql> insert into student(name) values('egon'); Query OK, 1 row affected (0.01 sec)mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | +----+------+------+ row in set (0.00 sec) View Code #在創(chuàng)建完表后，修改自增字段的起始值 mysql> create table student(-> id int primary key auto_increment,-> name varchar(20),-> sex enum('male','female') default 'male'-> );mysql> alter table student auto_increment=3;mysql> show create table student; ....... ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mysql> insert into student(name) values('egon'); Query OK, 1 row affected (0.01 sec)mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 3 | egon | male | +----+------+------+ row in set (0.00 sec)mysql> show create table student; ....... ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8#也可以創(chuàng)建表時指定auto_increment的初始值，注意初始值的設(shè)置為表選項(xiàng)，應(yīng)該放到括號外 create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' )auto_increment=3;#設(shè)置步長 sqlserver：自增步長基于表級別create table t1（id int。。。）engine=innodb,auto_increment=2 步長=2 default charset=utf8mysql自增的步長：show session variables like 'auto_inc%';#基于會話級別set session auth_increment_increment=2 #修改會話級別的步長#基于全局級別的set global auth_increment_increment=2 #修改全局級別的步長（所有會話都生效）#！！！注意了注意了注意了！！！ If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored. 翻譯：如果auto_increment_offset的值大于auto_increment_increment的值，則auto_increment_offset的值會被忽略 比如：設(shè)置auto_increment_offset=3，auto_increment_increment=2mysql> set global auto_increment_increment=5; Query OK, 0 rows affected (0.00 sec)mysql> set global auto_increment_offset=3; Query OK, 0 rows affected (0.00 sec)mysql> show variables like 'auto_incre%'; #需要退出重新登錄 +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | +--------------------------+-------+create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' );mysql> insert into student(name) values('egon1'),('egon2'),('egon3'); mysql> select * from student; +----+-------+------+ | id | name | sex | +----+-------+------+ | 3 | egon1 | male | | 8 | egon2 | male | | 13 | egon3 | male | +----+-------+------+ 步長increment與起始偏移量offset:auto_increment_increment,auto_increment_offset

六 foreign key

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員工信息表有三個字段：工號 ?姓名 ?部門

公司有3個部門，但是有1個億的員工，那意味著部門這個字段需要重復(fù)存儲，部門名字越長，越浪費(fèi)

解決方法：

我們完全可以定義一個部門表

然后讓員工信息表關(guān)聯(lián)該表，如何關(guān)聯(lián)，即foreign key

#表類型必須是innodb存儲引擎，且被關(guān)聯(lián)的字段，即references指定的另外一個表的字段，必須是主鍵 create table department( id int primary key, name varchar(20) not null )engine=innodb;#dpt_id外鍵，關(guān)聯(lián)父表（department主鍵id），同步更新，同步刪除 create table employee( id int primary key, name varchar(20) not null, dpt_id int, constraint fk_name foreign key(dpt_id) references department(id) on delete cascade on update cascade )engine=innodb;#先往父表department中插入記錄 insert into department values (1,'歐德博愛技術(shù)有限事業(yè)部'), (2,'艾利克斯人力資源部'), (3,'銷售部');#再往子表employee中插入記錄 insert into employee values (1,'egon',1), (2,'alex1',2), (3,'alex2',2), (4,'alex3',2), (5,'李坦克',3), (6,'劉飛機(jī)',3), (7,'張火箭',3), (8,'林子彈',3), (9,'加特林',3) ;#刪父表department，子表employee中對應(yīng)的記錄跟著刪 mysql> delete from department where id=3; mysql> select * from employee; +----+-------+--------+ | id | name | dpt_id | +----+-------+--------+ | 1 | egon | 1 | | 2 | alex1 | 2 | | 3 | alex2 | 2 | | 4 | alex3 | 2 | +----+-------+--------+#更新父表department，子表employee中對應(yīng)的記錄跟著改 mysql> update department set id=22222 where id=2; mysql> select * from employee; +----+-------+--------+ | id | name | dpt_id | +----+-------+--------+ | 1 | egon | 1 | | 3 | alex2 | 22222 | | 4 | alex3 | 22222 | | 5 | alex1 | 22222 | +----+-------+--------+ 示范 表1 foreign key 表2 則表1的多條記錄對應(yīng)表2的一條記錄，即多對一利用foreign key的原理我們可以制作兩張表的多對多，一對一關(guān)系 多對多：表1的多條記錄可以對應(yīng)表2的一條記錄表2的多條記錄也可以對應(yīng)表1的一條記錄一對一：表1的一條記錄唯一對應(yīng)表2的一條記錄，反之亦然分析時，我們先從按照上面的基本原理去套，然后再翻譯成真實(shí)的意義，就很好理解了、、 輔助理解

三張表：出版社，作者信息，書

一對多（或多對一）：一個出版社可以出版多本書

　　關(guān)聯(lián)方式：foreign key

=====================多對一===================== create table press( id int primary key auto_increment, name varchar(20) );create table book( id int primary key auto_increment, name varchar(20), press_id int not null, foreign key(press_id) references press(id) on delete cascade on update cascade );insert into press(name) values ('北京工業(yè)地雷出版社'), ('人民音樂不好聽出版社'), ('知識產(chǎn)權(quán)沒有用出版社') ;insert into book(name,press_id) values ('九陽神功',1), ('九陰真經(jīng)',2), ('九陰白骨爪',2), ('獨(dú)孤九劍',3), ('降龍十巴掌',2), ('葵花寶典',3) ; View Code

多對多：一個作者可以寫多本書，一本書也可以有多個作者，雙向的一對多，即多對多

　　關(guān)聯(lián)方式：foreign key+一張新的表

=====================多對多===================== create table author( id int primary key auto_increment, name varchar(20) );#這張表就存放作者表與書表的關(guān)系，即查詢二者的關(guān)系查這表就可以了 create table author2book( id int not null unique auto_increment, author_id int not null, book_id int not null, constraint fk_author foreign key(author_id) references author(id) on delete cascade on update cascade, constraint fk_book foreign key(book_id) references book(id) on delete cascade on update cascade, primary key(author_id,book_id) );#插入四個作者，id依次排開 insert into author(name) values('egon'),('alex'),('yuanhao'),('wpq');#每個作者與自己的代表作如下 egon: 九陽神功 九陰真經(jīng) 九陰白骨爪 獨(dú)孤九劍 降龍十巴掌 葵花寶典 alex: 九陽神功 葵花寶典 yuanhao: 獨(dú)孤九劍 降龍十巴掌 葵花寶典 wpq: 九陽神功insert into author2book(author_id,book_id) values (1,1), (1,2), (1,3), (1,4), (1,5), (1,6), (2,1), (2,6), (3,4), (3,5), (3,6), (4,1) ; View Code

#兩張表：學(xué)生表和客戶表

一對一：一個學(xué)生是一個客戶，一個客戶有可能變成一個學(xué)校，即一對一的關(guān)系

　　關(guān)聯(lián)方式：foreign key+unique

#一定是student來foreign key表customer，這樣就保證了： #1 學(xué)生一定是一個客戶， #2 客戶不一定是學(xué)生，但有可能成為一個學(xué)生 create table customer( id int primary key auto_increment, name varchar(20) not null );create table student( id int primary key auto_increment, name varchar(20) not null, class_name varchar(20) not null default 'python自動化', level int default 1, customer_id int unique, #該字段一定要是唯一的 foreign key(customer_id) references customer(id) #外鍵的字段一定要保證unique on delete cascade on update cascade );#增加客戶 insert into customer(name) values ('李飛機(jī)'), ('王大炮'), ('守榴彈'), ('吳坦克'), ('贏火箭'), ('戰(zhàn)地雷') ;#增加學(xué)生 insert into student(name,customer_id) values ('李飛機(jī)',1), ('王大炮',2) ; View Code

練習(xí)：賬號信息表，用戶組，主機(jī)表，主機(jī)組

#用戶表 create table user( id int not null unique auto_increment, username varchar(20) not null, password varchar(50) not null, primary key(username,password) );insert into user(username,password) values ('root','123'), ('egon','456'), ('alex','alex3714') ;#用戶組表 create table usergroup( id int primary key auto_increment, groupname varchar(20) not null unique );insert into usergroup(groupname) values ('IT'), ('Sale'), ('Finance'), ('boss') ;#主機(jī)表 create table host( id int primary key auto_increment, ip char(15) not null unique default '127.0.0.1' );insert into host(ip) values ('172.16.45.2'), ('172.16.31.10'), ('172.16.45.3'), ('172.16.31.11'), ('172.10.45.3'), ('172.10.45.4'), ('172.10.45.5'), ('192.168.1.20'), ('192.168.1.21'), ('192.168.1.22'), ('192.168.2.23'), ('192.168.2.223'), ('192.168.2.24'), ('192.168.3.22'), ('192.168.3.23'), ('192.168.3.24') ;#業(yè)務(wù)線表 create table business( id int primary key auto_increment, business varchar(20) not null unique ); insert into business(business) values ('輕松貸'), ('隨便花'), ('大富翁'), ('窮一生') ;#建關(guān)系：user與usergroup create table user2usergroup( id int not null unique auto_increment, user_id int not null, group_id int not null, primary key(user_id,group_id), foreign key(user_id) references user(id), foreign key(group_id) references usergroup(id) );insert into user2usergroup(user_id,group_id) values (1,1), (1,2), (1,3), (1,4), (2,3), (2,4), (3,4) ;#建關(guān)系：host與business create table host2business( id int not null unique auto_increment, host_id int not null, business_id int not null, primary key(host_id,business_id), foreign key(host_id) references host(id), foreign key(business_id) references business(id) );insert into host2business(host_id,business_id) values (1,1), (1,2), (1,3), (2,2), (2,3), (3,4) ;#建關(guān)系：user與host create table user2host( id int not null unique auto_increment, user_id int not null, host_id int not null, primary key(user_id,host_id), foreign key(user_id) references user(id), foreign key(host_id) references host(id) );insert into user2host(user_id,host_id) values (1,1), (1,2), (1,3), (1,4), (1,5), (1,6), (1,7), (1,8), (1,9), (1,10), (1,11), (1,12), (1,13), (1,14), (1,15), (1,16), (2,2), (2,3), (2,4), (2,5), (3,10), (3,11), (3,12) ; View Code

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作業(yè)：

七、總結(jié)

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1 not null 與defaultcreate table student2( id int primary key auto_increment, name char(5), sex enum('male','female') not null default 'female' );insert into student2(name) values('alex');create table student3( id int primary key auto_increment, name char(5), age int not null default 30 );insert into student3(name) values('alex');2 unique #單列唯一 create table teacher( id int not null unique, name char(10) ); insert into teacher values(1,'egon'); insert into teacher values(1,'alex');#多列唯一 #255.255.255.255 create table services( id int primary key auto_increment, name char(10), host char(15), port int, constraint host_port unique(host,port) );insert into services values('ftp','192.168.20.17',8080); insert into services values('httpd','192.168.20.17',8081);#auto_increment_offset:偏移量 create table dep( id int primary key auto_increment, name char(10) ); insert into dep(name) values('IT'),('HR'),('SALE'),('Boss');create table dep1( id int primary key auto_increment, name char(10) )auto_increment=10; insert into dep1(name) values('IT'),('HR'),('SALE'),('Boss');#auto_increment_increment:步長 create table dep2( id int primary key auto_increment, name char(10) ); set session auto_increment_increment=2; #會話級，只對當(dāng)前會話有效 set global auto_increment_increment=2; #全局，對所有的會話都有效 insert into dep1(name) values('IT'),('HR'),('SALE'),('Boss');#auto_increment_offset:偏移量+auto_increment_increment:步長 注意：如果auto_increment_offset的值大于auto_increment_increment的值，則auto_increment_offset的值會被忽略 set session auto_increment_offset=2; set session auto_increment_increment=3; show variables like '%auto_in%';create table dep3( id int primary key auto_increment, name char(10) ); insert into dep3(name) values('IT'),('HR'),('SALE'),('Boss');#foreign key #！！！先建被關(guān)聯(lián)的表,并且被關(guān)聯(lián)的字段必須唯一 create table dep( id int primary key auto_increment, name varchar(50), comment varchar(100) );create table emp_info( id int primary key auto_increment, name varchar(20), dep_id int, constraint fk_depid_id foreign key(dep_id) references dep(id) on delete cascade on update cascade );#先給被關(guān)聯(lián)的表初始化記錄 insert into dep values (1,'歐德博愛技術(shù)有限事業(yè)部','說的好...'), (2,'艾利克斯人力資源部','招不到人'), (3,'銷售部','賣不出東西');insert into emp_info values (1,'egon',1), (2,'alex1',2), (3,'alex2',2), (4,'alex3',2), (5,'李坦克',3), (6,'劉飛機(jī)',3), (7,'張火箭',3), (8,'林子彈',3), (9,'加特林',3) ; View Code

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轉(zhuǎn)載于:https://www.cnblogs.com/zhangningyang/p/7481504.html

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