Mysql完整性约束

  1. 基本介绍

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    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

#===========说明===============
#1. 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值
#2. 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值
sex enum('male','female') not null default 'male'

#必须为正值（无符号） 不允许为空 默认是20
age int unsigned NOT NULL default 20 
# 3. 是否是key
主键 primary key
外键 foreign key
索引 (index,unique...)
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  2. not nul 与 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



create table tb1(
    nid int not null defalut 2,
    num int not null

);
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     1 mysql> create table t11(id int);# id字段默认可以为空
 2 Query OK, 0 rows affected (0.05 sec)
 3 
 4 mysql> desc t11;
 5 +-------+---------+------+-----+---------+-------+
 6 | Field | Type    | Null | Key | Default | Extra |
 7 +-------+---------+------+-----+---------+-------+
 8 | id    | int(11) | YES   |          | NULL    |       |
 9 +-------+---------+------+-----+---------+-------+
10 row in set (0.03 sec)
11 mysql> insert into t11 values(); #给t11表插一个空的值
12 Query OK, 1 row affected (0.00 sec)
13 
14 #查询结果如下
15 mysql> select * from t11;
16 +------+
17 | id   |
18 +------+
19 | NULL |
20 +------+
21 row in set (0.00 sec)
22 
23 默认值可以为空
    验证
     1 mysql> create table t12(id int not null);#设置字段id不为空
 2 Query OK, 0 rows affected (0.03 sec)
 3 
 4 mysql> desc t12;
 5 +-------+---------+------+-----+---------+-------+
 6 | Field | Type    | Null | Key | Default | Extra |
 7 +-------+---------+------+-----+---------+-------+
 8 | id    | int(11) | NO   |     | NULL    |       |
 9 +-------+---------+------+-----+---------+-------+
10 row in set (0.01 sec)
11 
12 mysql> insert into t12 values();#不能插入空
13 ERROR 1364 (HY000): Field 'id' doesn't have a default value
14 
15 设置not null，插入值时不能为空
    设置not null，插入值时不能为空
     1 # 第一种情况
 2 mysql> create table t13(id int default 1);
 3 Query OK, 0 rows affected (0.03 sec)
 4 
 5 mysql> desc t13;
 6 +-------+---------+------+-----+---------+-------+
 7 | Field | Type    | Null | Key | Default | Extra |
 8 +-------+---------+------+-----+---------+-------+
 9 | id    | int(11) | YES  |           | 1              |              |
10 +-------+---------+------+-----+---------+-------+
11 row in set (0.01 sec)
12 
13 mysql> insert into t13 values();
14 Query OK, 1 row affected (0.00 sec)
15 
16 mysql> select * from t13;
17 +------+
18 | id   |
19 +------+
20 |    1  |
21 +------+
22 row in set (0.00 sec)
23 
24 
25 # 第二种情况
26 mysql> create table t14(id int not null default 2);
27 Query OK, 0 rows affected (0.02 sec)
28 
29 mysql> desc t14;
30 +-------+---------+------+-----+---------+-------+
31 | Field | Type    | Null | Key | Default | Extra |
32 +-------+---------+------+-----+---------+-------+
33 | id    | int(11) | NO      |         | 2               |             |
34 +-------+---------+------+-----+---------+-------+
35 row in set (0.01 sec)
36 
37 mysql> select * from t14;
38 +----+
39 | id |
40 +----+
41 |  2 |
42 +----+
43 row in set (0.00 sec)
44 
45 设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值
    设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值

     

  3. unique

    1. 单列唯一
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      #使用约束条件unique，来对公司部门的字段进行设置。
#第一种创建unique的方式
#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');
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    2. 联合唯一
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      # 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       |             |
+-------+----------+------+-----+---------+-------+
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 |
+------+---------------+------+
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'
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  4. primary key

     介绍

     

    1. 单列主键
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      # 创建t14表，为id字段设置主键，唯一的不同的记录
create table t14(
    id int primary key,
    name char(16)
);

insert into t14 values
(1,'xiaoma'),
(2,'xiaohong');

mysql> insert into t14 values(2,'wxxx');
ERROR 1062 (23000): Duplicate entry '6' for key 'PRIMARY'


#   not null + unique的化学反应，相当于给id设置primary key
create table t15(
    id int not null unique,
    name char(16)
);
mysql> create table t15(
    -> id int not null unique,
    -> name char(16)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> desc t15;
+-------+----------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id        | int(11)  | NO     | PRI | NULL       |             |
| name   | char(16) | YES  |         | NULL       |             |
+-------+----------+------+-----+---------+-------+
rows in set (0.02 sec)
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    2. 复合主键
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      create table t16(
    ip char(15),
    port int,
    primary key(ip,port)
);

insert into t16 values
('1.1.1.2',80),
('1.1.1.2',81);

验证复合主键的使用
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  5. 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
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    # 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)

不指定id，则自动增长
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  6.  1 mysql> insert into student values(4,'asb','female');
 2 Query OK, 1 row affected (0.00 sec)
 3 
 4 mysql> insert into student values(7,'wsb','female');
 5 Query OK, 1 row affected (0.01 sec)
 6 
 7 mysql> select * from student;
 8 +----+--------+--------+
 9 | id | name   | sex    |
10 +----+--------+--------+
11 |  1 | 老白   | male   |
12 |  2 | 小白   | male   |
13 |  4 | asb    | female |
14 |  7 | wsb    | female |
15 +----+--------+--------+
16 rows in set (0.00 sec)
17 
18 # 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
19 mysql>  insert into student(name) values ('大白');
20 Query OK, 1 row affected (0.00 sec)
21 
22 mysql> select * from student;
23 +----+--------+--------+
24 | id | name   | sex    |
25 +----+--------+--------+
26 |  1 | 老白   | male   |
27 |  2 | 小白   | male   |
28 |  4 | asb    | female |
29 |  7 | wsb    | female |
30 |  8 | 大白   | male   |
31 +----+--------+--------+
32 rows in set (0.00 sec)
33 
34 也可以指定id
    也可以指定id
     1 mysql> delete from student;
 2 Query OK, 5 rows affected (0.00 sec)
 3 
 4 mysql> select * from student;
 5 Empty set (0.00 sec)
 6 
 7 mysql> select * from student;
 8 Empty set (0.00 sec)
 9 
10 mysql> insert into student(name) values('ysb');
11 Query OK, 1 row affected (0.01 sec)
12 
13 mysql> select * from student;
14 +----+------+------+
15 | id | name | sex  |
16 +----+------+------+
17 |  9 | ysb  | male |
18 +----+------+------+
19 row in set (0.00 sec)
20 
21 #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
22 mysql> truncate student;
23 Query OK, 0 rows affected (0.03 sec)
24 
25 mysql>  insert into student(name) values('xiaobai');
26 Query OK, 1 row affected (0.00 sec)
27 
28 mysql> select * from student;
29 +----+---------+------+
30 | id | name    | sex  |
31 +----+---------+------+
32 |  1 | xiaobai | male |
33 +----+---------+------+
34 row in set (0.00 sec)
35 
36 mysql>
37 
38 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
    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

     

  7. 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。

  8. foreign key

    1. 快速理解foreign key

      之前创建表的时候都是在一张表中添加记录，比如如下表：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。

    2. 解决方法

      1. 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

        我们可以将上表改为如下结构：

        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）
      2. 创建两张表操作：
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        #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) 
);

#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 =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 |
+----+----------+-----+--------+
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             |
+----+-----------+----------------------+
rows in set (0.00 sec)
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      3. 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

        接下来将刚建好的两张表全部删除，先删除关联表（emp）,再删除被关联表（dep）

        接下来：
        重复上面的操作建表
        注意：在关联表中加入
        on delete cascade #同步删除
        on update cascade #同步更新

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        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
);
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      4. 接下来的操作，就复合我们正常的生活中的情况了。
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        #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             |
+----+-----------+----------------------+
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 |
+----+----------+-----+--------+
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             |
+-----+-----------+----------------------+
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 |
+----+----------+-----+--------+
rows in set (0.00 sec)
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posted @ 2020-09-06 18:56  Οo白麒麟оΟ  阅读(184)  评论(0)  编辑  收藏  举报