为了防止不符合规范的数据进入数据库，在用户对数据进行插入、修改、删除等 *** 作时，DBMS自动按照一定的约束条件对数据进行监测，使不符合规范的数据不能进入数据库，以确保数据库中存储的数据正确、有效、相容。
约束条件与数据类型的宽度一样，都是可选参数，主要分为以下几种：
| 约束条件 | 解释 |
|---|---|
| NOT NulL | 非空约束，指定某列不能为空； |
| UNIQUE | 唯一约束，指定某列或者几列组合不能重复 |
| PRIMARY KEY | 主键，指定该列的值可以唯一地标识该列记录 |
| FOREIGN KEY | 外键，指定该行记录从属于主表中的一条记录，主要用于参照完整性 |
是否可空，null表示空，非字符串
not null - 不可空
null - 可空
2.1 not null实例MysqL> create table t12 (ID int not null);query OK,0 rows affected (0.02 sec)MysqL> select * from t12;Empty set (0.00 sec)MysqL> desc t12;+-------+---------+------+-----+---------+-------+| FIEld | Type | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| ID | int(11) | NO | | NulL | |+-------+---------+------+-----+---------+-------+row in set (0.00 sec)#不能向ID列插入空元素。 MysqL> insert into t12 values (null);ERROR 1048 (23000): Column 'ID' cannot be nullMysqL> insert into t12 values (1);query OK,1 row affected (0.01 sec)我们约束某一列不为空，如果这一列中经常有重复的内容，就需要我们频繁的插入，这样会给我们的 *** 作带来新的负担，于是就出现了默认值的概念。
默认值，创建列时可以指定默认值，当插入数据时如果未主动设置，则自动添加默认值。
2.3 not null + default 示例MysqL> create table t13 (ID1 int not null,ID2 int not null default 222);query OK,0 rows affected (0.01 sec)MysqL> desc t13;+-------+---------+------+-----+---------+-------+| FIEld | Type | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| ID1 | int(11) | NO | | NulL | || ID2 | int(11) | NO | | 222 | |+-------+---------+------+-----+---------+-------+rows in set (0.01 sec)# 只向ID1字段添加值，会发现ID2字段会使用默认值填充MysqL> insert into t13 (ID1) values (111);query OK,1 row affected (0.00 sec)MysqL> select * from t13;+-----+-----+| ID1 | ID2 |+-----+-----+| 111 | 222 |+-----+-----+row in set (0.00 sec)# ID1字段不能为空，所以不能单独向ID2字段填充值；MysqL> insert into t13 (ID2) values (223);ERROR 1364 (HY000): FIEld 'ID1' doesn't have a default value# 向ID1，ID2中分别填充数据，ID2的填充数据会覆盖默认值MysqL> insert into t13 (ID1,ID2) values (112,223);query OK,1 row affected (0.00 sec)MysqL> select * from t13;+-----+-----+| ID1 | ID2 |+-----+-----+| 111 | 222 || 112 | 223 |+-----+-----+rows in set (0.00 sec)设置严格模式： 不支持对not null字段插入null值 不支持对自增长字段插入”值 不支持text字段有默认值直接在MysqL中生效(重启失效):MysqL>set sql_mode="STRICT_TRANS_tableS,NO_auto_CREATE_USER,NO_ENGINE_SUBSTITUTION";配置文件添加(永久失效)：sql-mode="STRICT_TRANS_tableS,NO_ENGINE_SUBSTITUTION"唯一约束，指定某列或者几列组合不能重复。
3.1 unique示例方法一：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),unique(name));MysqL> insert into department1 values(1,'IT','技术');query OK,1 row affected (0.00 sec)MysqL> insert into department1 values(1,'技术');ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'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)create table service(ID int primary key auto_increment,host varchar(15) not null,port int not null,unique(host,port) #联合唯一);MysqL> insert into service values -> (1,'Nginx','192.168.0.10',80),-> (2,'haproxy','192.168.0.20',-> (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',80);ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'主键为了保证表中的每一条数据的该字段都是表格中的唯一值。换言之，它是用来独一无二地确认一个表格中的每一行数据。
主键可以包含一个字段或多个字段。当主键包含多个栏位时，称为组合键 (Composite Key),也可以叫联合主键。
主键可以在建置新表格时设定 (运用 CREATE table 语句)，或是以改变现有的表格架构方式设定 (运用 ALTER table)。
主键必须唯一，主键值非空；可以是单一字段，也可以是多字段组合。
============单列做主键===============#方法一：not null+uniquecreate table department1(ID int not null unique,#主键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：在某一个字段后用primary keycreate table department2(ID int primary key,#主键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：在所有字段后单独定义primary keycreate table department3(ID int,primary key(ID); #创建主键并为其命名pk_nameMysqL> 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)# 方法四：给已经建成的表添加主键约束MysqL> create table department4( -> ID int,-> name varchar(20),-> comment varchar(100));query OK,0 rows affected (0.01 sec)MysqL> desc department4;+---------+--------------+------+-----+---------+-------+| FIEld | Type | Null | Key | Default | Extra |+---------+--------------+------+-----+---------+-------+| ID | int(11) | YES | | NulL | || name | varchar(20) | YES | | NulL | || comment | varchar(100) | YES | | NulL | |+---------+--------------+------+-----+---------+-------+rows in set (0.01 sec)MysqL> alter table department4 modify ID int primary key;query OK,0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0MysqL> desc department4;+---------+--------------+------+-----+---------+-------+| 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)==================多列做主键================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','mariadb') -> ;query OK,2 rows affected (0.00 sec)Records: 2 Duplicates: 0 Warnings: 0MysqL> insert into service values ('172.16.45.10','Nginx');ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'约束字段为自动增长，被约束的字段必须同时被key约束
4.3.1 设置auto_increment#不指定ID，则自动增长create table student(ID int primary key auto_increment,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 -> ('nick'),-> ('tank') -> ;MysqL> select * from student;+----+------+------+| ID | name | sex |+----+------+------+| 1 | nick | male || 2 | tank | male |+----+------+------+#也可以指定IDMysqL> insert into student values(4,'asb','female');query OK,1 row affected (0.00 sec)MysqL> insert into student values(7,'wsb',1 row affected (0.00 sec)MysqL> select * from student;+----+------+--------+| ID | name | sex |+----+------+--------+| 1 | nick | male || 2 | tank | male || 4 | asb | female || 7 | wsb | female |+----+------+--------+#对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长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 |+----+------+------+#应该用truncate清空表，比起delete一条一条地删除记录，truncate是直接清空表，在删除大表时用它MysqL> truncate student;query OK,0 rows affected (0.01 sec)MysqL> insert into student(name) values('nick');query OK,1 row affected (0.01 sec)MysqL> select * from student;+----+------+------+| ID | name | sex |+----+------+------+| 1 | nick | male |+----+------+------+row in set (0.00 sec)#在创建完表后，修改自增字段的起始值MysqL> create table student( -> ID int primary key auto_increment,-> 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('nick');query OK,1 row affected (0.01 sec)MysqL> select * from student;+----+------+------+| ID | name | sex |+----+------+------+| 3 | nick | male |+----+------+------+row in set (0.00 sec)MysqL> show create table student;.......ENGINE=InnoDB auto_INCREMENT=4 DEFAulT CHARSET=utf8#也可以创建表时指定auto_increment的初始值，注意初始值的设置为表选项，应该放到括号外create table student(ID int primary key auto_increment,'female') default 'male')auto_increment=3;#设置步长sqlserver：自增步长 基于表级别 create table t1（ ID int。。。 ）engine=innodb,auto_increment=2 步长=2 default charset=utf8MysqL自增的步长： show session variables like 'auto_inc%'; #基于会话级别 set session auth_increment_increment=2 #修改会话级别的步长 #基于全局级别的 set global auth_increment_increment=2 #修改全局级别的步长（所有会话都生效）#！！！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. 翻译：如果auto_increment_offset的值大于auto_increment_increment的值，则auto_increment_offset的值会被忽略 ，这相当于第一步步子就迈大了，扯着了蛋比如：设置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%'; #需要退出重新登录+--------------------------+-------+| Variable_name | Value |+--------------------------+-------+| auto_increment_increment | 1 || auto_increment_offset | 1 |+--------------------------+-------+create table student(ID int primary key auto_increment,'female') default 'male');MysqL> insert into student(name) values('nick1'),('nick2'),('nick3');MysqL> select * from student;+----+-------+------+| ID | name | sex |+----+-------+------+| 3 | nick1 | male || 8 | nick2 | male || 13 | nick3 | male |+----+-------+------+步长:auto_increment_increment,起始偏移量:auto_increment_offset多表 ：
假设我们要描述所有公司的员工，需要描述的属性有这些 ： 工号 姓名 部门
公司有3个部门，但是有1个亿的员工，那意味着部门这个字段需要重复存储，部门名字越长，越浪费
解决方法： 我们完全可以定义一个部门表 然后让员工信息表关联该表，如何关联，即foreign key
5.1 创造外键的条件MysqL> create table departments (dep_ID int(4),dep_name varchar(11));query OK,0 rows affected (0.02 sec)MysqL> desc departments;+----------+-------------+------+-----+---------+-------+| FIEld | Type | Null | Key | Default | Extra |+----------+-------------+------+-----+---------+-------+| dep_ID | int(4) | YES | | NulL | || dep_name | varchar(11) | YES | | NulL | |+----------+-------------+------+-----+---------+-------+rows in set (0.00 sec)# 创建外键不成功MysqL> create table staff_info (s_ID int,dep_ID int,foreign key(dep_ID) references departments(dep_ID));ERROR 1215 (HY000): Cannot add foreign key # 设置dep_ID非空，仍然不能成功创建外键MysqL> alter table departments modify dep_ID int(4) not null;query OK,0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0MysqL> desc departments;+----------+-------------+------+-----+---------+-------+| FIEld | Type | Null | Key | Default | Extra |+----------+-------------+------+-----+---------+-------+| dep_ID | int(4) | NO | | NulL | || dep_name | varchar(11) | YES | | NulL | |+----------+-------------+------+-----+---------+-------+rows in set (0.00 sec)MysqL> create table staff_info (s_ID int,foreign key(dep_ID) references departments(dep_ID));ERROR 1215 (HY000): Cannot add foreign key constraint# 当设置字段为unique唯一字段时，设置该字段为外键成功MysqL> alter table departments modify dep_ID int(4) unique;query OK,0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0MysqL> desc departments; +----------+-------------+------+-----+---------+-------+| FIEld | Type | Null | Key | Default | Extra |+----------+-------------+------+-----+---------+-------+| dep_ID | int(4) | YES | UNI | NulL | || dep_name | varchar(11) | YES | | NulL | |+----------+-------------+------+-----+---------+-------+rows in set (0.01 sec)MysqL> create table staff_info (s_ID int,foreign key(dep_ID) references departments(dep_ID));query OK,0 rows affected (0.02 sec)#表类型必须是innodb存储引擎，且被关联的字段，即references指定的另外一个表的字段，必须保证唯一create table department(ID int primary key,name varchar(20) not null)engine=innodb;#dpt_ID外键，关联父表（department主键ID），同步更新，同步删除create table employee(ID int primary key,name varchar(20) not null,dpt_ID int,foreign key(dpt_ID)references department(ID)on delete cascade # 级连删除on update cascade # 级连更新)engine=innodb;#先往父表department中插入记录insert into department values(1,'教质部'),(2,'技术部'),(3,'人力资源部');#再往子表employee中插入记录insert into employee values(1,'jason',1),'oscar',2),'nick',(4,'tank',(5,'mac',3),(6,'李沁洋',(7,'皮卡丘',(8,'程咬金',(9,'程咬银',3);#删父表department，子表employee中对应的记录跟着删MysqL> delete from department where ID=2;query OK,1 row affected (0.00 sec)MysqL> select * from employee;+----+-----------+--------+| ID | name | dpt_ID |+----+-----------+--------+| 1 | jason | 1 || 5 | mac | 3 || 6 | 李沁洋 | 3 || 7 | 皮卡丘 | 3 || 8 | 程咬金 | 3 || 9 | 程咬银 | 3 |+----+-----------+--------+rows in set (0.00 sec)#更新父表department，子表employee中对应的记录跟着改MysqL> update department set ID=2 where ID=3;query OK,1 row affected (0.01 sec)Rows matched: 1 Changed: 1 Warnings: 0MysqL> select * from employee;+----+-----------+--------+| ID | name | dpt_ID |+----+-----------+--------+| 1 | jason | 1 || 5 | mac | 2 || 6 | 李沁洋 | 2 || 7 | 皮卡丘 | 2 || 8 | 程咬金 | 2 || 9 | 程咬银 | 2 |+----+-----------+--------+rows in set (0.00 sec). cascade方式在父表上update/delete记录时，同步update/delete掉子表的匹配记录 . set null方式在父表上update/delete记录时，将子表上匹配记录的列设为null要注意子表的外键列不能为not null . No action方式如果子表中有匹配的记录,则不允许对父表对应候选键进行update/delete *** 作 . Restrict方式同no action,都是立即检查外键约束. Set default方式父表有变更时,子表将外键列设置成一个默认的值 但Innodb不能识别以上是内存溢出为你收集整理的MySQL表的完整性约束全部内容，希望文章能够帮你解决MySQL表的完整性约束所遇到的程序开发问题。
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