数据库----mysql表的约束和查询

一,mysql中表的约束

　 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：

  1. NOT NULL ：非空约束，指定某列不能为空；

    • 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)

  2. 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)

    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"

  3. UNIQUE : 唯一约束，指定某列或者几列组合不能重复

    • 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,'IT','技术');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

    • not null 和 unique的结合

      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)

    • 联合唯一

      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'

  4. PRIMARY KEY ：

    • 主键，指定该列的值可以唯一地标识该列记录

    • 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)

      • 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'

  5. 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

    • 创建外键的条件
    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    |       |
+----------+-------------+------+-----+---------+-------+
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)

    • 外键操作示例

      #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 |
+----+-----------+--------+
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 |
+----+-----------+--------+
rows in set (0.00 sec)

外键操作示例
  6. 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)

二,表结构的查询

  • 单表查询

  1. 单表查询语法

    SELECT DISTINCT 字段1,字段2... FROM 表名
                              WHERE 条件
                              GROUP BY field
                              HAVING 筛选
                              ORDER BY field
                              LIMIT 限制条数

  2. 关键字执行的优先级

    关键字 作用
    from 找到表
    where 拿着where指定的约束条件,去文件/表中取出一条条记录
    group by 将取出的一条条记录进行分组group by,如果没有group by,则整体作为一组
    select distinct 执行select(去重)
    having 将分组的结果进行having过滤
    order by 将结果按条件排序：order by
    limit 限制结果的显示条数

  3. 简单查询

        SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

    SELECT * FROM employee;

    SELECT emp_name,salary FROM employee;

#避免重复DISTINCT
    SELECT DISTINCT post FROM employee;    

#通过四则运算查询
    SELECT emp_name, salary*12 FROM employee;
    SELECT emp_name, salary*12 AS Annual_salary FROM employee;
    SELECT emp_name, salary*12 Annual_salary FROM employee;

#定义显示格式
   CONCAT() 函数用于连接字符串
   SELECT CONCAT('姓名: ',emp_name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一个参数为分隔符
   SELECT CONCAT_WS(':',emp_name,salary*12)  AS Annual_salary 
   FROM employee;

   结合CASE语句：
   SELECT
       (
           CASE
           WHEN emp_name = 'jingliyang' THEN
               emp_name
           WHEN emp_name = 'alex' THEN
               CONCAT(emp_name,'_BIGSB')
           ELSE
               concat(emp_name, 'SB')
           END
       ) as new_name
   FROM
       employee;

  4. where约束

    select 字段名 from 表名 where 条件

    类型 用法
    比较运算符 > < >= <= <> !=
    between 80 and 100 值在80到100之间(包含80,100)
    in(80,90,100) 值是80或90或100
    like 'e%' 通配符可以是%或_,%表示任意多字符, _表示一个字符
    regexp 正则匹配
    is / is not is null / is not null
    逻辑运算符 and or not 
    #1:单条件查询
    SELECT emp_name FROM employee
        WHERE post='sale';
        
#2:多条件查询
    SELECT emp_name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

#3:关键字BETWEEN AND
    SELECT emp_name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;
    
#4:关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS)
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment=''; 注意''是空字符串，不是null
    ps：
        执行
        update employee set post_comment='' where id=2;
        再用上条查看，就会有结果了

#5:关键字IN集合查询
    SELECT emp_name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT emp_name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;

#6:关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'eg%';

    通配符’_’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'al__';

三,表结构

  1. 删除表
drop table 表名;
  1. 多表结构的创建和分析
分析步骤：
# 1,先站在左表的角度去找
是否左表的多条记录可以对应右表的一条记录,如果是,则证明左表的一个字段foreign key 右表一个字段(通常是id)

# 2,再站在右表的角度去找
是否右表的多条记录可以对应左表的一条记录,如果是,则证明右表的一个字段foreign key 左表一个字段(通常是id)

# 3,总结：
# 多对一：
如果只有步骤1成立,则是左表多对一右表
如果只有步骤2成立,则是右表多对一左表

# 多对多
如果步骤1和2同时成立,则证明这两张表时一个双向的多对一,即多对多,需要定义一个这两张表的关系表来专门存放二者的关系

# 一对一:
如果1和2都不成立,而是左表的一条记录唯一对应右表的一条记录,反之亦然.这种情况很简单,就是在左表foreign key右表的基础上,将左表的外键字段设置成unique即可

# 一对多或多对一 示例
mysql> create table press(
    -> id int primary key auto_increment,
    -> name varchar(20)
    -> );
mysql> create table book(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> press_id int not null,
    -> foreign key(press_id) references press(id)
    -> on delete cascade
    -> on update cascade
    -> );
mysql> insert into press(name) values
    -> ('北京工业地雷出版社'),
    -> ('人民音乐不好听出版社'),
    -> ('知识产权没有用出版社');
mysql> insert into book(name,press_id) values
    -> ('九阳神功',1),
    -> ('九阴真经',2),
    -> ('九阴白骨爪',2),
    -> ('独孤九剑',3),
    -> ('降龙十巴掌',2),
    -> ('葵花宝典',3);
mysql> select * from book;
+----+-----------------+----------+
| id | name            | press_id |
+----+-----------------+----------+
|  1 | 九阳神功         |        1 |
|  2 | 九阴真经         |        2 |
|  3 | 九阴白骨爪       |        2 |
|  4 | 独孤九剑         |        3 |
|  5 | 降龙十巴掌       |        2 |
|  6 | 葵花宝典         |        3 |
+----+-----------------+----------+

# 多对多
# 三张表：作者,书,对应关系
# 多对多：一个作者可以写多本书,一本书也可以有多个作者,双向的一对多,即多对多
# 关联方式：foreign key+一张新的表
# 作者表
mysql>  create table author(
    ->  id int primary key auto_increment,
    ->  name varchar(20)
    -> );
# 这张表就存放作者表与书表的关系，即查询二者的关系查这表就可以了
mysql> create table author_book(
    -> id int primary key auto_increment,
    -> author_id int not null,
    -> book_id int not null,
    -> constraint fk_author foreign key(author_id) references author(id)
    -> on delete cascade on update cascade,
    -> constraint fk_book foreign key(book_id) references book(id)
    -> on delete cascade on update cascade,
    -> unique(author_id,book_id)
    -> );

mysql> insert into author(name) values
    -> ('孙悟空'),('猪八戒'),('唐僧'),('沙和尚');
mysql> insert into author_book(author_id,book_id) values
    -> (1,4),(1,2),(1,3),(2,1),(2,6),(3,5),(4,2),(4,6);
    
mysql> select * from book;
+----+-----------------+----------+
| id | name            | press_id |
+----+-----------------+----------+
|  1 | 九阳神功        |        1 |
|  2 | 九阴真经        |        2 |
|  3 | 九阴白骨爪      |        2 |
|  4 | 独孤九剑        |        3 |
|  5 | 降龙十巴掌      |        2 |
|  6 | 葵花宝典        |        3 |
+----+-----------------+----------+
mysql> select * from author;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | 孙悟空    |
|  2 | 猪八戒    |
|  3 | 唐僧      |
|  4 | 沙和尚    |
+----+-----------+
mysql> select * from author_book;
+----+-----------+---------+
| id | author_id | book_id |
+----+-----------+---------+
|  2 |         1 |       2 |
|  3 |         1 |       3 |
|  1 |         1 |       4 |
|  4 |         2 |       1 |
|  5 |         2 |       6 |
|  6 |         3 |       5 |
|  7 |         4 |       2 |
|  8 |         4 |       6 |
+----+-----------+---------+


# 一对一
# 两张表：用户表和博客表
# 一对一：一个用户只有一个博客
# 关联方式：foreign key+unique
# 用户表
mysql> create table user(
    -> id int primary key auto_increment,
    -> name varchar(12));
mysql> insert into user(name) values
    -> ('孙悟空'),('猪八戒'),('沙悟净');
# 博客表
mysql> create table blog(
    -> id int primary key auto_increment,
    -> url varchar(40) not null unique,
    -> name_id int unique,
    -> foreign key(name_id) references user(id)
    -> on delete cascade on update cascade);
mysql> insert into blog(url,name_id) values
    -> ('www.baidu.com',1),('www.sohu.com',3),('www.taobao.com',2);
mysql> select * from user;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | 孙悟空    |
|  2 | 猪八戒    |
|  3 | 沙悟净    |
+----+-----------+
mysql> select * from blog;
+----+----------------+---------+
| id | url            | name_id |
+----+----------------+---------+
|  1 | www.baidu.com  |       1 |
|  2 | www.sohu.com   |       3 |
|  3 | www.taobao.com |       2 |
+----+----------------+---------+
posted @ 2019-12-06 10:10  豆子V  阅读(950)  评论(0)  编辑  收藏  举报