一 介紹

約束條件與數(shù)據(jù)類(lèi)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(shí)該字段為該表的主鍵，可以唯一的標(biāo)識(shí)記錄 FOREIGN KEY (FK) 標(biāo)識(shí)該字段為該表的外鍵 NOT NULL 標(biāo)識(shí)該字段不能為空 UNIQUE KEY (UK) 標(biāo)識(shí)該字段的值是唯一的 AUTO_INCREMENT 標(biāo)識(shí)該字段的值自動(dòng)增長(zhǎng)（整數(shù)類(lèi)型，而且為主鍵） DEFAULT 為該字段設(shè)置默認(rèn)值UNSIGNED 無(wú)符號(hào) ZEROFILL 使用0填充

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說(shuō)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(shí)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(wú)符號(hào)） 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

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(shí)可以指定默認(rèn)值，當(dāng)插入數(shù)據(jù)時(shí)如果未主動(dòng)設(shè)置，則自動(dòng)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(wú)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

============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 | | +-------+---------+------+-----+---------+-------+ 1 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

primary key字段的值不為空且唯一

一個(gè)表中可以：

單列做主鍵
多列做主鍵（復(fù)合主鍵）

但一個(gè)表內(nèi)只能有一個(gè)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(gè)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 | | +--------------+-------------+------+-----+---------+-------+ 3 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

約束字段為自動(dòng)增長(zhǎng)，被約束的字段必須同時(shí)被key約束

#不指定id，則自動(dòng)增長(zhǎng) 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(duì)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(zhǎng) 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(shí)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 | +----+------+------+ 1 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(shí)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(hào)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(zhǎng) sqlserver：自增步長(zhǎng)基于表級(jí)別create table t1（id int。。。）engine=innodb,auto_increment=2 步長(zhǎng)=2 default charset=utf8mysql自增的步長(zhǎng)：show session variables like 'auto_inc%';#基于會(huì)話級(jí)別set session auth_increment_increment=2 #修改會(huì)話級(jí)別的步長(zhǎng)#基于全局級(jí)別的set global auth_increment_increment=2 #修改全局級(jí)別的步長(zhǎng)（所有會(huì)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(huì)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(zhǎng)increment與起始偏移量offset:auto_increment_increment,auto_increment_offset

六 foreign key

一 快速理解foreign key

員工信息表有三個(gè)字段：工號(hào) ?姓名 ?部門(mén)

公司有3個(gè)部門(mén)，但是有1個(gè)億的員工，那意味著部門(mén)這個(gè)字段需要重復(fù)存儲(chǔ)，部門(mén)名字越長(zhǎng)，越浪費(fèi)

解決方法：

我們完全可以定義一個(gè)部門(mén)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(lèi)型必須是innodb存儲(chǔ)引擎，且被關(guān)聯(lián)的字段，即references指定的另外一個(gè)表的字段，必須保證唯一 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(ài)技術(shù)有限事業(yè)部'), (2,'艾利克斯人力資源部'), (3,'銷(xiāo)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(duì)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(duì)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?

二?如何找出兩張表之間的關(guān)系?

分析步驟： #1、先站在左表的角度去找 是否左表的多條記錄可以對(duì)應(yīng)右表的一條記錄，如果是，則證明左表的一個(gè)字段foreign key 右表一個(gè)字段（通常是id）#2、再站在右表的角度去找 是否右表的多條記錄可以對(duì)應(yīng)左表的一條記錄，如果是，則證明右表的一個(gè)字段foreign key 左表一個(gè)字段（通常是id）#3、總結(jié)： #多對(duì)一： 如果只有步驟1成立，則是左表多對(duì)一右表 如果只有步驟2成立，則是右表多對(duì)一左表#多對(duì)多 如果步驟1和2同時(shí)成立，則證明這兩張表時(shí)一個(gè)雙向的多對(duì)一，即多對(duì)多,需要定義一個(gè)這兩張表的關(guān)系表來(lái)專(zhuān)門(mén)存放二者的關(guān)系#一對(duì)一: 如果1和2都不成立，而是左表的一條記錄唯一對(duì)應(yīng)右表的一條記錄，反之亦然。這種情況很簡(jiǎn)單，就是在左表foreign key右表的基礎(chǔ)上，將左表的外鍵字段設(shè)置成unique即可

三 建立表之間的關(guān)系

#一對(duì)多或稱(chēng)為多對(duì)一 三張表：出版社，作者信息，書(shū)一對(duì)多（或多對(duì)一）：一個(gè)出版社可以出版多本書(shū)關(guān)聯(lián)方式：foreign key =====================多對(duì)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(lè)不好聽(tīng)出版社'), ('知識(shí)產(chǎn)權(quán)沒(méi)有用出版社') ;insert into book(name,press_id) values ('九陽(yáng)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 一夫多妻制#妻子表的丈夫id外鍵到丈夫表的id 其他例子

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#多對(duì)多 三張表：出版社，作者信息，書(shū)多對(duì)多：一個(gè)作者可以寫(xiě)多本書(shū)，一本書(shū)也可以有多個(gè)作者，雙向的一對(duì)多，即多對(duì)多關(guān)聯(lián)方式：foreign key+一張新的表 =====================多對(duì)多===================== create table author( id int primary key auto_increment, name varchar(20) );#這張表就存放作者表與書(shū)表的關(guān)系，即查詢(xún)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(gè)作者，id依次排開(kāi) insert into author(name) values('egon'),('alex'),('yuanhao'),('wpq');#每個(gè)作者與自己的代表作如下 1 egon: 1 九陽(yáng)神功2 九陰真經(jīng)3 九陰白骨爪4 獨(dú)孤九劍5 降龍十巴掌6 葵花寶典2 alex: 1 九陽(yáng)神功6 葵花寶典3 yuanhao:4 獨(dú)孤九劍5 降龍十巴掌6 葵花寶典4 wpq:1 九陽(yáng)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 單張表：用戶(hù)表+相親關(guān)系表，相當(dāng)于：用戶(hù)表+相親關(guān)系表+用戶(hù)表 多張表：用戶(hù)表+用戶(hù)與主機(jī)關(guān)系表+主機(jī)表中間那一張存放關(guān)系的表，對(duì)外關(guān)聯(lián)的字段可以聯(lián)合唯一 其他例子

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#一對(duì)一 兩張表：學(xué)生表和客戶(hù)表一對(duì)一：一個(gè)學(xué)生是一個(gè)客戶(hù)，一個(gè)客戶(hù)有可能變成一個(gè)學(xué)校，即一對(duì)一的關(guān)系關(guān)聯(lián)方式：foreign key+unique #一定是student來(lái)foreign key表customer，這樣就保證了： #1 學(xué)生一定是一個(gè)客戶(hù)， #2 客戶(hù)不一定是學(xué)生，但有可能成為一個(gè)學(xué)生 create table customer( id int primary key auto_increment, name varchar(20) not null, qq varchar(10) not null, phone char(16) not null );create table student( id int primary key auto_increment, class_name varchar(20) not null, 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(hù) insert into customer(name,qq,phone) values ('李飛機(jī)','31811231',13811341220), ('王大炮','123123123',15213146809), ('守榴彈','283818181',1867141331), ('吳坦克','283818181',1851143312), ('贏火箭','888818181',1861243314), ('戰(zhàn)地雷','112312312',18811431230) ;#增加學(xué)生 insert into student(class_name,customer_id) values ('脫產(chǎn)3班',3), ('周末19期',4), ('周末19期',5) ; View Code 例一：一個(gè)用戶(hù)只有一個(gè)博客用戶(hù)表：id name1 egon2 alex3 wupeiqi博客表 fk+uniqueid url name_id1 xxxx 12 yyyy 33 zzz 2例二：一個(gè)管理員唯一對(duì)應(yīng)一個(gè)用戶(hù)用戶(hù)表：id user password1 egon xxxx2 alex yyyy管理員表：fk+uniqueid user_id password1 1 xxxxx2 2 yyyyy

轉(zhuǎn)載于:https://www.cnblogs.com/fu-yong/p/8495003.html

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