mysql完整性约束实例详解
本文实例讲述了mysql完整性约束。分享给大家供大家参考,具体如下:
主要内容
- not null 与 default
- unique
- primary
- auto_increment
- foreign key
约束条件作用:用于保证数据的完整性和一致性
主要分为
primary key (pk) #标识该字段为该表的主键,可以唯一的标识记录
foreign key (fk) #标识该字段为该表的外键
not null #标识该字段不能为空
unique key (uk) #标识该字段的值是唯一的,
auto_increment #标识该字段的值自动增长(整数类型,而且为主键)
default #为该字段设置默认值
unsigned #无符号
zerofill #使用0填充
unique
在mysql中称为单列唯一
#例子1: create table department( id int, name char(10) unique ); mysql> insert into department values(1,'it'),(2,'it'); error 1062 (23000): duplicate entry 'it' for key 'name' #例子2: create table department( id int unique, name char(10) unique ); insert into department values(1,'it'),(2,'sale'); #第二种创建unique的方式 create table department( id int, name char(10) , unique(id), unique(name) ); insert into department values(1,'it'),(2,'sale');
联合唯一:只要两列记录,有一列不同,既符合联合唯一的约束
# 创建services表 mysql> create table services( -> id int, -> ip char(15), -> port int, -> unique(id), -> unique(ip,port) -> ); query ok, 0 rows affected (0.05 sec) mysql> desc services; +-------+----------+------+-----+---------+-------+ | field | type | null | key | default | extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | yes | uni | null | | | ip | char(15) | yes | mul | null | | | port | int(11) | yes | | null | | +-------+----------+------+-----+---------+-------+ 3 rows in set (0.01 sec) #联合唯一,只要两列记录,有一列不同,既符合联合唯一的约束 mysql> insert into services values -> (1,'192,168,11,23',80), -> (2,'192,168,11,23',81), -> (3,'192,168,11,25',80); query ok, 3 rows affected (0.01 sec) records: 3 duplicates: 0 warnings: 0 mysql> select * from services; +------+---------------+------+ | id | ip | port | +------+---------------+------+ | 1 | 192,168,11,23 | 80 | | 2 | 192,168,11,23 | 81 | | 3 | 192,168,11,25 | 80 | +------+---------------+------+ 3 rows in set (0.00 sec) mysql> insert into services values (4,'192,168,11,23',80); error 1062 (23000): duplicate entry '192,168,11,23-80' for key 'ip'
auto_increment
约束:约束的字段为自动增长,约束的字段必须同时被key约束
不指定id,则自动增长
# 创建student 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 | | +-------+-----------------------+------+-----+---------+----------------+ rows in set (0.17 sec) #插入记录 mysql> insert into student(name) values ('老白'),('小白'); query ok, 2 rows affected (0.01 sec) records: 2 duplicates: 0 warnings: 0 mysql> select * from student; +----+--------+------+ | id | name | sex | +----+--------+------+ | 1 | 老白 | male | | 2 | 小白 | male | +----+--------+------+ rows in set (0.00 sec)
指定id的情况
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 (0.01 sec) mysql> select * from student; +----+--------+--------+ | id | name | sex | +----+--------+--------+ | 1 | 老白 | male | | 2 | 小白 | male | | 4 | asb | female | | 7 | wsb | female | +----+--------+--------+ rows in set (0.00 sec) # 再次插入一条不指定id的记录,会在之前的最后一条记录继续增长 mysql> insert into student(name) values ('大白'); query ok, 1 row affected (0.00 sec) mysql> select * from student; +----+--------+--------+ | id | name | sex | +----+--------+--------+ | 1 | 老白 | male | | 2 | 小白 | male | | 4 | asb | female | | 7 | wsb | female | | 8 | 大白 | male | +----+--------+--------+ rows in set (0.00 sec)
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, 5 rows affected (0.00 sec) mysql> select * from student; empty set (0.00 sec) mysql> select * from student; empty set (0.00 sec) mysql> insert into student(name) values('ysb'); query ok, 1 row affected (0.01 sec) mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 9 | ysb | male | +----+------+------+ row in set (0.00 sec) #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 (0.03 sec) mysql> insert into student(name) values('xiaobai'); query ok, 1 row affected (0.00 sec) mysql> select * from student; +----+---------+------+ | id | name | sex | +----+---------+------+ | 1 | xiaobai | male | +----+---------+------+ row in set (0.00 sec) mysql> auto_increment_increment和 auto_increment_offset
查看可用的 开头auto_inc的词
mysql> show variables like 'auto_inc%'; +--------------------------+-------+ | variable_name | value | +--------------------------+-------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | +--------------------------+-------+ rows in set (0.02 sec)
# 步长auto_increment_increment,默认为1 # 起始的偏移量auto_increment_offset, 默认是1 # 设置步长 为会话设置,只在本次连接中有效 set session auto_increment_increment=5; #全局设置步长 都有效。 set global auto_increment_increment=5; # 设置起始偏移量 set global auto_increment_offset=3;
强调: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
设置完起始偏移量和步长之后,再次执行show variables like'auto_inc%';
发现跟之前一样,必须先exit,再登录才有效。
mysql> show variables like'auto_inc%'; +--------------------------+-------+ | variable_name | value | +--------------------------+-------+ | auto_increment_increment | 5 | | auto_increment_offset | 3 | +--------------------------+-------+ rows in set (0.00 sec) #因为之前有一条记录id=1 mysql> select * from student; +----+---------+------+ | id | name | sex | +----+---------+------+ | 1 | xiaobai | male | +----+---------+------+ row in set (0.00 sec) # 下次插入的时候,从起始位置3开始,每次插入记录id+5 mysql> insert into student(name) values('ma1'),('ma2'),('ma3'); query ok, 3 rows affected (0.00 sec) records: 3 duplicates: 0 warnings: 0 mysql> select * from student; +----+---------+------+ | id | name | sex | +----+---------+------+ | 1 | xiaobai | male | | 3 | ma1 | male | | 8 | ma2 | male | | 13 | ma3 | male | +----+---------+------+
清空表区分delete和truncate的区别:
delete from t1; #如果有自增id,新增的数据,仍然是以删除前的最后一样作为起始。
truncate table t1;数据量大,删除速度比上一条快,且直接从零开始。
foreign key
理解foreign key
如上图如果一个公司有很多员工,每个员工都对应一个部门,在填表的时候就会重复写这些部门,太冗余了
我们可以将它们分离
此时有两张表,一张是employee表,简称emp表(关联表,也就从表)。一张是department表,简称dep表(被关联表,也叫主表)。
#1.创建表时先创建被关联表,再创建关联表 # 先创建被关联表(dep表) create table dep( id int primary key, name varchar(20) not null, descripe varchar(20) not null ); #再创建关联表(emp表) create table emp( id int primary key, name varchar(20) not null, age int not null, dep_id int, constraint fk_dep foreign key(dep_id) references dep(id) //创建约束 ); #2.插入记录时,先往被关联表中插入记录,再往关联表中插入记录 insert into dep values (1,'it','it技术有限部门'), (2,'销售部','销售部门'), (3,'财务部','花钱太多部门'); insert into emp values (1,'zhangsan',18,1), (2,'lisi',19,1), (3,'egon',20,2), (4,'yuanhao',40,3), (5,'alex',18,2);
3.删除表
#按道理来说,删除了部门表中的某个部门,员工表的有关联的记录相继删除。 mysql> delete from dep where id=3; error 1451 (23000): cannot delete or update a parent row: a foreign key constraint fails (`db5`.`emp`, constraint `fk_name` foreign key (`dep_id`) references `dep` (`id`)) #但是先删除员工表的记录之后,再删除当前部门就没有任何问题 mysql> delete from emp where dep_id =3; query ok, 1 row affected (0.00 sec) mysql> select * from emp; +----+----------+-----+--------+ | id | name | age | dep_id | +----+----------+-----+--------+ | 1 | zhangsan | 18 | 1 | | 2 | lisi | 18 | 1 | | 3 | egon | 20 | 2 | | 5 | alex | 18 | 2 | +----+----------+-----+--------+ 4 rows in set (0.00 sec) mysql> delete from dep where id=3; query ok, 1 row affected (0.00 sec) mysql> select * from dep; +----+-----------+----------------------+ | id | name | descripe | +----+-----------+----------------------+ | 1 | it | it技术有限部门 | | 2 | 销售部 | 销售部门 | +----+-----------+----------------------+ 2 rows in set (0.00 sec)
上面的删除表记录的操作比较繁琐,按道理讲,裁掉一个部门,该部门的员工也会被裁掉。其实呢,在建表的时候还有个很重要的内容,叫同步删除,同步更新
on delete cascade #同步删除
on update cascade #同步更新
create table emp( id int primary key, name varchar(20) not null, age int not null, dep_id int, constraint fk_dep foreign key(dep_id) references dep(id) on delete cascade #同步删除 on update cascade #同步更新 );
#再去删被关联表(dep)的记录,关联表(emp)中的记录也跟着删除 mysql> delete from dep where id=3; query ok, 1 row affected (0.00 sec) mysql> select * from dep; +----+-----------+----------------------+ | id | name | descripe | +----+-----------+----------------------+ | 1 | it | it技术有限部门 | | 2 | 销售部 | 销售部门 | +----+-----------+----------------------+ 2 rows in set (0.00 sec) mysql> select * from emp; +----+----------+-----+--------+ | id | name | age | dep_id | +----+----------+-----+--------+ | 1 | zhangsan | 18 | 1 | | 2 | lisi | 19 | 1 | | 3 | egon | 20 | 2 | | 5 | alex | 18 | 2 | +----+----------+-----+--------+ 4 rows in set (0.00 sec) #再去更改被关联表(dep)的记录,关联表(emp)中的记录也跟着更改 mysql> update dep set id=222 where id=2; query ok, 1 row affected (0.02 sec) rows matched: 1 changed: 1 warnings: 0 # 赶紧去查看一下两张表是否都被删除了,是否都被更改了 mysql> select * from dep; +-----+-----------+----------------------+ | id | name | descripe | +-----+-----------+----------------------+ | 1 | it | it技术有限部门 | | 222 | 销售部 | 销售部门 | +-----+-----------+----------------------+ 2 rows in set (0.00 sec) mysql> select * from emp; +----+----------+-----+--------+ | id | name | age | dep_id | +----+----------+-----+--------+ | 1 | zhangsan | 18 | 1 | | 2 | lisi | 19 | 1 | | 3 | egon | 20 | 222 | | 5 | alex | 18 | 222 | +----+----------+-----+--------+ 4 rows in set (0.00 sec)
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