MySql

完整性约束

1. not null 与 default

• 如果单独设置not null 不能插入空值

• 如果即设置了not null,又指定default,可以插入空值,会走default

(1)默认值可以为空

mysql> create table t1(id int);# id字段默认可以为空
Query OK, 0 rows affected (0.05 sec)

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES   |          | NULL    |       |
+-------+---------+------+-----+---------+-------+
row in set (0.03 sec)
mysql> insert into t1 values(); #给t1表插一个空的值
Query OK, 1 row affected (0.00 sec)
#查询结果如下
mysql> select * from t1;
+------+
| id   |
+------+
| NULL |
+------+
row in set (0.00 sec)
默认值可以为空

(2)设置not null后 ,插入值不能为空

#给 num设置不能为空
mysql> create table t2(num int not null);
Query OK, 0 rows affected (0.47 sec)

mysql> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)

#此时报错,显示不能插空值
mysql> insert into t2 values();
ERROR 1364 (HY000): Field 'num' doesn't have a default value

(3) 如果即设置了not null,又指定default,可以插入空值,值为default,当插入值不是空值,结果是自己的值

#创建 t3表此时设置not null,并且default为 4
mysql> create table t3(num int not null default 4);
Query OK, 0 rows affected (0.46 sec)

mysql> desc t3;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | int(11) | NO   |     | 4       |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)
#当插入数据为空时,此时是默认值生效
mysql> insert into t3 values();
Query OK, 1 row affected (0.11 sec)
#但是也可以插入值但是此时却是给的值而不是默认值
mysql> insert into t3 values(2);
Query OK, 1 row affected (0.11 sec)

mysql> select * from db2.t3;
+-----+
| num |
+-----+
|   4 |
|   2 |
+-----+
2 rows in set (0.00 sec)

2. unique key

(1)单列唯一

# 创建t4表,给name设置单列唯一
mysql> create table t4(id int not null,name char(20) unique);
Query OK, 0 rows affected (0.47 sec)

mysql> desc t4;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | NO   |     | NULL    |       |
| name  | char(20) | YES  | UNI | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> insert into t4 values(1,'alex');
Query OK, 1 row affected (0.35 sec)

mysql> insert into t4 values(1,'alex');
#有于给name设置了单列为一,因此插入相同的值会报错
ERROR 1062 (23000): Duplicate entry 'alex' for key 'name'
#当将值改为与第一次不同时候,发现此时插入成功
mysql> insert into t4 values(1,'alex2');
Query OK, 1 row affected (0.34 sec)
mysql> select * from db2.t4;
+----+-------+
| id | name  |
+----+-------+
|  1 | alex  |
|  1 | alex2 |
+----+-------+
2 rows in set (0.00 sec)

两种书写方式:

create table t4(
    id int not null,
    name char(20) unique
);

create table t4(
    id int not null,
    name char(20),
    unique(name)
);
insert into t4(id,name) values(1,'alex');
insert into t4(id,name) values(1,'wusir');

(2)多列唯一:只要有一列是相同的就不能插入.

#书写
create table t5(
    id int,
    name char(20),
    unique(id),
    unique(name)
);

#创建文件t5 ,此时将id 和name都设置 unique
mysql> create table t5(
    ->     id int,
    ->     name char(20),
    ->     unique(id),
    ->     unique(name)
    -> );
Query OK, 0 rows affected (0.58 sec)
#插入一组值
mysql> insert into t5 values(2,'alex');
ERROR 1062 (23000): Duplicate entry 'alex' for key 'name'
#插入有相同内容的值时候报错
mysql> insert into t5 values(1,'alex2');
ERROR 1062 (23000): Duplicate entry '1' for key 'id'
#当所有内容都不同时才能插入
mysql> insert into t5 values(2,'alex2');
Query OK, 1 row affected (0.10 sec)

mysql> desc t5;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | YES  | UNI | NULL    |       |
| name  | char(20) | YES  | UNI | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> select * from db2.t5;
+------+-------+
| id   | name  |
+------+-------+
|    1 | alex  |
|    2 | alex2 |
+------+-------+
2 rows in set (0.00 sec)

(3)联合唯一:多列相同时不能插入

**适用场景  学生选课

#书写
create table t6(
    id int,
    name char(20),
    unique(id,name) 
);

#创建t6表 ,设置联合唯一
mysql> create table t6(
    -> id int,
    ->     name char(20),
    ->     unique(id,name)
    -> );
Query OK, 0 rows affected (0.47 sec)

mysql> desc t6;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | YES  | MUL | NULL    |       |
| name  | char(20) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> insert into t6 values(1,"alex");
Query OK, 1 row affected (0.35 sec)
#当插入相同的两列记录会报错
mysql> insert into t6 values(1,"alex");
ERROR 1062 (23000): Duplicate entry '1-alex' for key 'id'
mysql> insert into t6 values(2,"alex");
Query OK, 1 row affected (0.11 sec)
#但是当两列记录中的一项内容不同就可以插入
mysql> insert into t6 values(1,"wusir");
Query OK, 1 row affected (0.10 sec)

mysql> select *from t6;
+------+-------+
| id   | name  |
+------+-------+
|    1 | alex  |
|    1 | wusir |
|    2 | alex  |
+------+-------+
3 rows in set (0.01 sec)

3.  主键primary key  (not null + unique 的结果)

(1)单列主键 不能为空 ,并且是唯一

#书写
# primary key 索引(针对于大量数据) 查询速度要快
create table t7(
    id int primary key,
    name varchar(10) unique
);
#相同结果的写法
create table t8(
    id int not null unique,
    name varchar(10) unique
);

#创建t7表 此时为id设置主键,并且 name唯一
mysql> create table t7(
    ->     id int primary key,
    ->     name varchar(10) unique
    -> );
Query OK, 0 rows affected (0.51 sec)

mysql> desc t7;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(10) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
#当插入空值时候回报错
mysql> insert into t7 values();
ERROR 1364 (HY000): Field 'id' doesn't have a default value

mysql> insert into t7(id,name) values(1,"wusir");
Query OK, 1 row affected (0.37 sec)
#id唯一且不能为空
mysql> insert into t7(id,name) values(1,"wusir");
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into t7(id,name) values(1,"wusir2");
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into t7(id,name) values(2,"wusir2");
Query OK, 1 row affected (0.35 sec)

mysql> select *from db2.t7;
+----+--------+
| id | name   |
+----+--------+
|  1 | wusir  |
|  2 | wusir2 |
+----+--------+
2 rows in set (0.00 sec)

(2)联合主键

#书写方式
create table t9(    
    id int,    
    name varchar(10),
    primary key(id,name)
);

#创建了t9表,此时可以设置空值
mysql> create table t9(
    ->     id int,
    ->     name varchar(10),
    ->     primary key(id,name)
    -> );
Query OK, 0 rows affected (0.30 sec)

mysql> desc t9;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | 0       |       |
| name  | varchar(10) | NO   | PRI |         |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> insert into t9 values();
Query OK, 1 row affected (0.11 sec)

mysql> insert into t9(id name) values(1,"alex");
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'name) values(1,"alex")' at line 1
mysql> insert into t9(id, name) values(1,"alex");
Query OK, 1 row affected (0.10 sec)

mysql>  insert into t9(id, name) values(1,"alex2");
Query OK, 1 row affected (0.11 sec)

mysql> select *from db2.t9;
+----+-------+
| id | name  |
+----+-------+
|  0 |       |
|  1 | alex  |
|  1 | alex2 |
+----+-------+
3 rows in set (0.00 sec)

**学生选课可以为空

4.auto_increment (自增长)

#书写方式
create table student(
    id int primary key auto_increment,
    name varchar(20) not null,
    sex enum('male','female') default 'male', 
    ip varchar(20) unique
);

insert into student(name,sex,ip) values ('alex','female','127.0.0.5'),('wusir','male','173.45.32.1');

mysql> create table student(
    ->     id int primary key auto_increment,
    ->     name varchar(20) not null,
    ->     sex enum('male','female') default 'male',
    ->     ip varchar(20) unique
    -> );
Query OK, 0 rows affected (0.23 sec)
mysql> insert into student(name,sex,ip) values ('alex','female','127.0.0.5'),('wusir','male','173.45.32.1');
Query OK, 2 rows affected (0.37 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> desc student;
+-------+-----------------------+------+-----+---------+----------------+
| Field | Type                  | Null | Key | Default | Extra          |
+-------+-----------------------+------+-----+---------+----------------+
| id    | int(11)               | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20)           | NO   |     | NULL    |                |
| sex   | enum('male','female') | YES  |     | male    |                |
| ip    | varchar(20)           | YES  | UNI | NULL    |                |
+-------+-----------------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)

mysql> select *from student;
+----+-------+--------+-------------+
| id | name  | sex    | ip          |
+----+-------+--------+-------------+
|  1 | alex  | female | 127.0.0.5   |
|  2 | wusir | male   | 173.45.32.1 |
+----+-------+--------+-------------+
2 rows in set (0.00 sec)

涉及到删除时:

#当删除第二条记录时候,会发现此时的结果任然保留id为2 时的序号
mysql> delete from student where id =2;
Query OK, 1 row affected (0.49 sec)

mysql> select *from student;
+----+-------+--------+------------+
| id | name  | sex    | ip         |
+----+-------+--------+------------+
|  1 | alex  | female | 127.0.0.5  |
|  3 | alex1 | male   | 127.1.0.12 |
+----+-------+--------+------------+
2 rows in set (0.00 sec)
#在插入记录时候,会在后续排列
mysql> insert into student(name,sex,ip) values ('alex3','male','127.1.5.72');
Query OK, 1 row affected (0.11 sec)

mysql>  select *from student;
+----+-------+--------+------------+
| id | name  | sex    | ip         |
+----+-------+--------+------------+
|  1 | alex  | female | 127.0.0.5  |
|  3 | alex1 | male   | 127.1.0.12 |
|  4 | alex3 | male   | 127.1.5.72 |
+----+-------+--------+------------+
3 rows in set (0.00 sec)
#但是可以指定id进行插入
mysql> insert into student(id,name,sex,ip) values (2,'wusir','male','127.3.5.9');
Query OK, 1 row affected (0.11 sec)

mysql> mysql> Ctrl-C -- exit!
 select *from student;
+----+-------+--------+------------+
| id | name  | sex    | ip         |
+----+-------+--------+------------+
|  1 | alex  | female | 127.0.0.5  |
|  2 | wusir | male   | 127.3.5.9  |
|  3 | alex1 | male   | 127.1.0.12 |
|  4 | alex3 | male   | 127.1.5.72 |
+----+-------+--------+------------+
4 rows in set (0.00 sec)
#清空记录
mysql> delete from student;
Query OK, 4 rows affected (0.36 sec)

mysql>  delete from student;
Query OK, 0 rows affected (0.00 sec)

mysql> show tables;
+---------------+
| Tables_in_db2 |
+---------------+
| student       |
| t1            |
| t2            |
| t3            |
| t4            |
| t5            |
| t6            |
| t7            |
| t9            |
+---------------+
9 rows in set (0.11 sec)
#但是会保留自增长的序号,后续插入
mysql> insert into student(name,sex,ip) values ('alex3','male','127.1.5.72');
Query OK, 1 row affected (0.36 sec)

mysql>  select *from student;
+----+-------+------+------------+
| id | name  | sex  | ip         |
+----+-------+------+------------+
|  5 | alex3 | male | 127.1.5.72 |
+----+-------+------+------------+
1 row in set (0.00 sec)
#但是使用truncate时候,不会保留增长的序号
mysql> truncate table student;
Query OK, 0 rows affected (0.21 sec)

mysql> show tables;
+---------------+
| Tables_in_db2 |
+---------------+
| student       |
| t1            |
| t2            |
| t3            |
| t4            |
| t5            |
| t6            |
| t7            |
| t9            |
+---------------+
9 rows in set (0.00 sec)

mysql> select *from student;
Empty set (0.00 sec)

mysql> insert into student(name,sex,ip) values ('alex3','male','127.1.5.72');
Query OK, 1 row affected (0.35 sec)
#再次插入时候从一开始
mysql> select *from student;
+----+-------+------+------------+
| id | name  | sex  | ip         |
+----+-------+------+------------+
|  1 | alex3 | male | 127.1.5.72 |
+----+-------+------+------------+
1 row in set (0.00 sec)

*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。

5.  foreign key(外键)

 # 先创建主表
mysql> create table dep(
    -> id int primary key auto_increment,
    ->     name char(10) unique,
    ->     dep_desc varchar(50) not null
    -> );
Query OK, 0 rows affected (0.47 sec)

#创建从表
mysql> create table emp(
    ->     eid int primary key auto_increment,
    ->     name char(10) not null,
    ->     age int not null,
    ->     dep_id int,
    ->     school_id int,
    ->     constraint fk_dep foreign key(dep_id) references dep(id) );
Query OK, 0 rows affected (0.55 sec)
#主表插入数据
mysql> insert into dep(name,dep_desc) values('校长部','校长管理有限部门'),('公关部','公关管理有限部门'),('IT部门','IT技 术有限部门'),('财务部','管钱很多部门');
Query OK, 4 rows affected (0.11 sec)
Records: 4  Duplicates: 0  Warnings: 0
#从表插入数据
mysql> insert into emp(name,age,dep_id)
    -> values
    -> ('alex',18,1),
    -> ('wusir',30,2),
    -> ('吴老板',20,3),
    -> ('马老板',18,4),
    -> ('邱老板',20,2),
    -> ('女神',16,3);
Query OK, 6 rows affected (0.06 sec)
Records: 6  Duplicates: 0  Warnings: 0
#将从表中对应主表的id为2 的项删除
mysql> delete from emp where dep_id =2;
Query OK, 2 rows affected (0.37 sec)

mysql> select *from emp;
+-----+-----------+-----+--------+-----------+
| eid | name      | age | dep_id | school_id |
+-----+-----------+-----+--------+-----------+
|   1 | alex      |  18 |      1 |      NULL |
|   3 | 吴老板    |  20 |      3 |      NULL |
|   4 | 马老板    |  18 |      4 |      NULL |
|   6 | 女神      |  16 |      3 |      NULL |
+-----+-----------+-----+--------+-----------+
4 rows in set (0.00 sec)
#此时可以将主表中id为2的部门删除
mysql> delete from dep where id=2;
Query OK, 1 row affected (0.35 sec)

mysql> select *from dep;
+----+-----------+--------------------------+
| id | name      | dep_desc                 |
+----+-----------+--------------------------+
|  1 | 校长部    | 校长管理有限部门         |
|  3 | IT部门    | IT技术有限部门           |
|  4 | 财务部    | 管钱很多部门             |
+----+-----------+--------------------------+
3 rows in set (0.00 sec)
#但是若果直接删除,会报错,因为此时该id对应的存在内容
mysql> delete from dep where id=3;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`db2`.`emp`, CONSTRAINT `fk_dep` FOREIGN KEY (`dep_id`) REFERENCES `dep` (`id`))

所以要同步删除和同步更新

#当在创建从表时添加    on delete cascade   on update cascade 同步更新同步删除
mysql> select *from dep;
+----+-----------+--------------------------+
| id | name      | dep_desc                 |
+----+-----------+--------------------------+
|  1 | 校长部    | 校长管理有限部门         |
|  2 | 公关部    | 公关管理有限部门         |
|  3 | IT部门    | IT技术有限部门           |
|  4 | 财务部    | 管钱很多部门             |
+----+-----------+--------------------------+
4 rows in set (0.00 sec)
#直接删除部门可以,而对应的部门内的人也删除
mysql> delete from dep where id=2;
Query OK, 1 row affected (0.35 sec)

mysql> select *from dep;
+----+-----------+--------------------------+
| id | name      | dep_desc                 |
+----+-----------+--------------------------+
|  1 | 校长部    | 校长管理有限部门         |
|  3 | IT部门    | IT技术有限部门           |
|  4 | 财务部    | 管钱很多部门             |
+----+-----------+--------------------------+
3 rows in set (0.00 sec)

mysql> select *from emp;
+-----+-----------+-----+--------+-----------+
| eid | name      | age | dep_id | school_id |
+-----+-----------+-----+--------+-----------+
|   1 | alex      |  18 |      1 |      NULL |
|   3 | 吴老板    |  20 |      3 |      NULL |
|   4 | 马老板    |  18 |      4 |      NULL |
|   6 | 女神      |  16 |      3 |      NULL |
+-----+-----------+-----+--------+-----------+
4 rows in set (0.00 sec)