mysql—表的完整性约束
mysql表的完整性约束
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概览
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、删除等操作时,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:
# 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
NOT NULL
是否可空,null表示空,非字符串
not null - 不可空
null - 可空
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 | |+-------+---------+------+-----+---------+-------+1 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 null mysql> insert into t12 values (1); Query OK, 1 row affected (0.01 sec)
not null示例
DEFAULT
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,这样会给我们的操作带来新的负担,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
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 | |+-------+---------+------+-----+---------+-------+2 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 |+-----+-----+1 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 | +-----+-----+ 2 rows in set (0.00 sec)
not null + default 示例
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_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"
not null不生效
UNIQUE
唯一约束,指定某列或者几列组合不能重复
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,‘IT‘,‘技术‘); ERROR 1062 (23000): Duplicate entry ‘IT‘ for key ‘name‘
unique示例
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 和unique的结合
create table service( id int primary key auto_increment, name varchar(20), 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‘,80),-> (3,‘mysql‘,‘192.168.0.30‘,3306)-> ; Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> 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‘
联合唯一
PRIMARY KEY
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。
1.单字段主键
============单列做主键=============== #方法一:not null+unique create 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 key create 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 key create table department3( id int, name varchar(20), comment varchar(100),primary key(id); #创建主键并为其命名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) 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 | |+---------+--------------+------+-----+---------+-------+3 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: 0 mysql> desc department4;+---------+--------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+---------+--------------+------+-----+---------+-------+| id | int(11) | NO | PRI | NULL | || name | varchar(20) | YES | | NULL | || comment | varchar(100) | YES | | NULL | |+---------+--------------+------+-----+---------+-------+3 rows in set (0.01 sec)
单字段主键
2.多字段主键
==================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 | |+--------------+-------------+------+-----+---------+-------+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: 0 mysql> insert into service values (‘172.16.45.10‘,‘3306‘,‘nginx‘); ERROR 1062 (23000): Duplicate entry ‘172.16.45.10-3306‘ for key ‘PRIMARY‘
多字段主键
AUTO_INCREMENT
约束字段为自动增长,被约束的字段必须同时被key约束
#不指定id,则自动增长 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 |+----+------+--------+ 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(‘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)
设置auto_increment
了解知识
#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, -> 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=utf8 mysql> 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, name varchar(20), sex enum(‘male‘,‘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=utf8 mysql自增的步长: show session variables like ‘auto_inc%‘; #基于会话级别 </span><span style="color: #0000ff;">set</span> session auth_increment_increment<span style="color: #808080;">=</span><span style="color: #800000; font-weight: bold;">2</span><span style="color: #000000;"> #修改会话级别的步长 #基于全局级别的 </span><span style="color: #0000ff;">set</span> global auth_increment_increment<span style="color: #808080;">=</span><span style="color: #800000; font-weight: bold;">2</span><span style="color: #000000;"> #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=2 mysql> 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, 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 |+----+-------+------+ 步长:auto_increment_increment,起始偏移量:auto_increment_offset
offset偏移量
FOREIKEY
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
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 | |+----------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec) 创建外键不成功 mysql> create table staff_info (s_id int,name varchar(20),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: 0 mysql> desc departments;+----------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+----------+-------------+------+-----+---------+-------+| dep_id | int(4) | NO | | NULL | || dep_name | varchar(11) | YES | | NULL | |+----------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec) mysql> create table staff_info (s_id int,name varchar(20),dep_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: 0 mysql> desc departments; +----------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+----------+-------------+------+-----+---------+-------+| dep_id | int(4) | YES | UNI | NULL | || dep_name | varchar(11) | YES | | NULL | |+----------+-------------+------+-----+---------+-------+2 rows in set (0.01 sec) mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id)); Query OK, 0 rows affected (0.02 sec)
创造外键的条件
#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,‘yuan‘,1), (2,‘nezha‘,2), (3,‘egon‘,2), (4,‘alex‘,2), (5,‘wusir‘,3), (6,‘李沁洋‘,3), (7,‘皮卡丘‘,3), (8,‘程咬金‘,3), (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 | yuan | 1 || 5 | wusir | 3 || 6 | 李沁洋 | 3 || 7 | 皮卡丘 | 3 || 8 | 程咬金 | 3 || 9 | 程咬银 | 3 |+----+-----------+--------+6 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: 0 mysql> select * from employee;+----+-----------+--------+| id | name | dpt_id |+----+-----------+--------+| 1 | yuan | 1 || 5 | wusir | 2 || 6 | 李沁洋 | 2 || 7 | 皮卡丘 | 2 || 8 | 程咬金 | 2 || 9 | 程咬银 | 2 |+----+-----------+--------+6 rows in set (0.00 sec)
外键操作示例
. 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操作 . 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
on delete(了解)
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列1 类型1 约束1,update 表名set 列=值 where 条件;eg:update student name=‘李四‘ where id=1;delete from 表名 where 条件;