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數據庫----mysql表的約束和查詢

本文轉載自   查看原文   2019-12-06 10:10   326 

一,mysql中表的約束

　 為了防止不符合規范的數據進入數據庫，在用戶對數據進行插入、修改、刪除等操作時，DBMS自動按照一定的約束條件對數據進行監測，使不符合規范的數據不能進入數據庫，以確保數據庫中存儲的數據正確、有效、相容。

約束條件與數據類型的寬度一樣，都是可選參數，主要分為以下幾種：

  1. NOT NULL ：非空約束，指定某列不能為空；

    • 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: 我們約束某一列不為空，如果這一列中經常有重復的內容，就需要我們頻繁的插入，這樣會給我們的操作帶來新的負擔，於是就出現了默認值的概念。默認值，創建列時可以指定默認值，當插入數據時如果未主動設置，則自動添加默認值

    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，會發現id2字段會使用默認值填充
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字段不能為空，所以不能單獨向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中分別填充數據，id2的填充數據會覆蓋默認值
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)

    設置嚴格模式：
    不支持對not null字段插入null值
    不支持對自增長字段插入”值
    不支持text字段有默認值

直接在mysql中生效(重啟失效):
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 : 唯一約束，指定某列或者幾列組合不能重復

    • 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','技術');
Query OK, 1 row affected (0.00 sec)
mysql> insert into department1 values(1,'IT','技術');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

    • not null 和 unique的結合

      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)

    • 聯合唯一

      create table service(
id int primary key auto_increment,
name varchar(20),
host varchar(15) not null,
port int not null,
unique(host,port) #聯合唯一
);

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 ：

    • 主鍵，指定該列的值可以唯一地標識該列記錄

    • 主鍵可以包含一個字段或多個字段。當主鍵包含多個欄位時，稱為組合鍵 (Composite Key),也可以叫聯合主鍵。

    • 主鍵可以在建置新表格時設定 (運用 CREATE TABLE 語句)，或是以改變現有的表格架構方式設定 (運用 ALTER TABLE)。

    • 主鍵必須唯一，主鍵值非空；可以是單一字段，也可以是多字段組合。

      • 單字段主鍵

        ============單列做主鍵===============
#方法一：not null+unique
create table department1(
id int not null unique, #主鍵
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：在某一個字段后用primary key
create table department2(
id int primary key, #主鍵
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：在所有字段后單獨定義primary key
create table department3(
id int,
name varchar(20),
comment varchar(100),
primary key(id); #創建主鍵並為其命名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：給已經建成的表添加主鍵約束
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)

      • 多字段主鍵

        ==================多列做主鍵================
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 ：外鍵，指定該行記錄從屬於主表中的一條記錄，主要用於參照完整性

    • 創建外鍵的條件
    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)

# 創建外鍵不成功
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 

# 設置dep_id非空，仍然不能成功創建外鍵
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

# 當設置字段為unique唯一字段時，設置該字段為外鍵成功
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)

    • 外鍵操作示例

      #表類型必須是innodb存儲引擎，且被關聯的字段，即references指定的另外一個表的字段，必須保證唯一
create table department(
id int primary key,
name varchar(20) not null
)engine=innodb;

#dpt_id外鍵，關聯父表（department主鍵id），同步更新，同步刪除
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  # 級連刪除
on update cascade # 級連更新
)engine=innodb;

#先往父表department中插入記錄
insert into department values
(1,'教質部'),
(2,'技術部'),
(3,'人力資源部');

#再往子表employee中插入記錄
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,'程咬銀',3)
;

#刪父表department，子表employee中對應的記錄跟着刪
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 | 程咬銀    |      3 |
+----+-----------+--------+
rows in set (0.00 sec)

#更新父表department，子表employee中對應的記錄跟着改
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 | 程咬銀    |      2 |
+----+-----------+--------+
rows in set (0.00 sec)

外鍵操作示例
  6. AUTO_INCREMENT : 約束字段為自動增長，被約束的字段必須同時被key約束
    #不指定id，則自動增長
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 |
+----+------+--------+


#對於自增的字段，在用delete刪除后，再插入值，該字段仍按照刪除前的位置繼續增長
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 |
+----+------+------+

#應該用truncate清空表，比起delete一條一條地刪除記錄，truncate是直接清空表，在刪除大表時用它
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,表結構的查詢

  • 單表查詢

  1. 單表查詢語法

    SELECT DISTINCT 字段1,字段2... FROM 表名
                              WHERE 條件
                              GROUP BY field
                              HAVING 篩選
                              ORDER BY field
                              LIMIT 限制條數

  2. 關鍵字執行的優先級

    關鍵字 作用
    from 找到表
    where 拿着where指定的約束條件,去文件/表中取出一條條記錄
    group by 將取出的一條條記錄進行分組group by,如果沒有group by,則整體作為一組
    select distinct 執行select(去重)
    having 將分組的結果進行having過濾
    order by 將結果按條件排序：order by
    limit 限制結果的顯示條數

  3. 簡單查詢

        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;

#避免重復DISTINCT
    SELECT DISTINCT post FROM employee;    

#通過四則運算查詢
    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;

#定義顯示格式
   CONCAT() 函數用於連接字符串
   SELECT CONCAT('姓名: ',emp_name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一個參數為分隔符
   SELECT CONCAT_WS(':',emp_name,salary*12)  AS Annual_salary 
   FROM employee;

   結合CASE語句：
   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約束

    select 字段名 from 表名 where 條件

    類型 用法
    比較運算符 > < >= <= <> !=
    between 80 and 100 值在80到100之間(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, _表示一個字符
    regexp 正則匹配
    is / is not is null / is not null
    邏輯運算符 and or not 
    #1:單條件查詢
    SELECT emp_name FROM employee
        WHERE post='sale';
        
#2:多條件查詢
    SELECT emp_name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

#3:關鍵字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:關鍵字IS NULL(判斷某個字段是否為NULL不能用等號，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：
        執行
        update employee set post_comment='' where id=2;
        再用上條查看，就會有結果了

#5:關鍵字IN集合查詢
    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:關鍵字LIKE模糊查詢
    通配符’%’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'eg%';

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

三,表結構

  1. 刪除表
drop table 表名;
  1. 多表結構的創建和分析
分析步驟：
# 1,先站在左表的角度去找
是否左表的多條記錄可以對應右表的一條記錄,如果是,則證明左表的一個字段foreign key 右表一個字段(通常是id)

# 2,再站在右表的角度去找
是否右表的多條記錄可以對應左表的一條記錄,如果是,則證明右表的一個字段foreign key 左表一個字段(通常是id)

# 3,總結：
# 多對一：
如果只有步驟1成立,則是左表多對一右表
如果只有步驟2成立,則是右表多對一左表

# 多對多
如果步驟1和2同時成立,則證明這兩張表時一個雙向的多對一,即多對多,需要定義一個這兩張表的關系表來專門存放二者的關系

# 一對一:
如果1和2都不成立,而是左表的一條記錄唯一對應右表的一條記錄,反之亦然.這種情況很簡單,就是在左表foreign key右表的基礎上,將左表的外鍵字段設置成unique即可

# 一對多或多對一 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
    -> ('北京工業地雷出版社'),
    -> ('人民音樂不好聽出版社'),
    -> ('知識產權沒有用出版社');
mysql> insert into book(name,press_id) values
    -> ('九陽神功',1),
    -> ('九陰真經',2),
    -> ('九陰白骨爪',2),
    -> ('獨孤九劍',3),
    -> ('降龍十巴掌',2),
    -> ('葵花寶典',3);
mysql> select * from book;
+----+-----------------+----------+
| id | name            | press_id |
+----+-----------------+----------+
|  1 | 九陽神功         |        1 |
|  2 | 九陰真經         |        2 |
|  3 | 九陰白骨爪       |        2 |
|  4 | 獨孤九劍         |        3 |
|  5 | 降龍十巴掌       |        2 |
|  6 | 葵花寶典         |        3 |
+----+-----------------+----------+

# 多對多
# 三張表：作者,書,對應關系
# 多對多：一個作者可以寫多本書,一本書也可以有多個作者,雙向的一對多,即多對多
# 關聯方式：foreign key+一張新的表
# 作者表
mysql>  create table author(
    ->  id int primary key auto_increment,
    ->  name varchar(20)
    -> );
# 這張表就存放作者表與書表的關系，即查詢二者的關系查這表就可以了
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
    -> ('孫悟空'),('豬八戒'),('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 | 九陽神功        |        1 |
|  2 | 九陰真經        |        2 |
|  3 | 九陰白骨爪      |        2 |
|  4 | 獨孤九劍        |        3 |
|  5 | 降龍十巴掌      |        2 |
|  6 | 葵花寶典        |        3 |
+----+-----------------+----------+
mysql> select * from author;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | 孫悟空    |
|  2 | 豬八戒    |
|  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 |
+----+-----------+---------+


# 一對一
# 兩張表：用戶表和博客表
# 一對一：一個用戶只有一個博客
# 關聯方式：foreign key+unique
# 用戶表
mysql> create table user(
    -> id int primary key auto_increment,
    -> name varchar(12));
mysql> insert into user(name) values
    -> ('孫悟空'),('豬八戒'),('沙悟凈');
# 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 | 孫悟空    |
|  2 | 豬八戒    |
|  3 | 沙悟凈    |
+----+-----------+
mysql> select * from blog;
+----+----------------+---------+
| id | url            | name_id |
+----+----------------+---------+
|  1 | www.baidu.com  |       1 |
|  2 | www.sohu.com   |       3 |
|  3 | www.taobao.com |       2 |
+----+----------------+---------+


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