MySQL数据库约束,表的设计

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系列专栏:MySQL通关系列

文章目录

  • 数据库约束
    • not null
    • unique
    • default
    • primary key 主键
      • auto_increment
    • foreign key 外键
    • check
  • 表的设计
    • 一对一
    • 一对多
    • 多对多
      • 多对多建表实例
      • 插入数据到实例实现多对多

数据库约束

not null

  • 指定某列的存储不能为null值
 create table student (id int not null,name varchar(20));
Query OK, 0 rows affected (0.01 sec)

mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

unique

  • 保证某列必须有唯一的值,插入重复的值就会报错

default

  • 规定给列赋值时的默认值
create table student(id int,name varchar(20) default '匿名');

primary key 主键

  • 主键约束,是not null 与unique的结合,确保某列的赋值不能为null,并且是唯一的

auto_increment

  • 自增特点:
    1.如果表中没有记录,自增从1开始
    2.如果有数据,从上一条记录往下自增
    3.插入再删掉数据,自增的值不会重复利用,会按删掉的那条开始自增
 create table student (id int primary key auto_increment,name varchar(20));
Query OK, 0 rows affected (0.01 sec)

mysql> desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20) | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> insert into student values(null,'张三');
Query OK, 1 row affected (0.00 sec)

mysql> select * from student;
+----+--------+
| id | name   |
+----+--------+
|  1 | 张三   |
+----+--------+
1 row in set (0.00 sec)

foreign key 外键

  • 外键约束,在表一中的数据必须在表二中存在,要参照完整性准则
  • 外键约束描述的是两张表的两个列之间的“依赖关系”
  • 外键约束会影响表的删除,例如下面的实例的class表被关联,所以它不能被轻易删除
mysql> create table class (
    -> id int primary key,
    -> name varchar(20) not null
    -> );
Query OK, 0 rows affected (0.04 sec)

mysql> create table student (
    -> id int primary key,
    -> name varchar(20) not null,
    -> email varchar(20) default 'unknow',
    -> QQ varchar(20) unique,
    -> classId int , foreign key (classId) references class(id)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> desc class;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.02 sec)

mysql> desc student;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(11)     | NO   | PRI | NULL    |       |
| name    | varchar(20) | NO   |     | NULL    |       |
| email   | varchar(20) | YES  |     | unknow  |       |
| QQ      | varchar(20) | YES  | UNI | NULL    |       |
| classId | int(11)     | YES  | MUL | NULL    |       |
+---------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

check

  • 指定一个条件,通过条件来对值进行判定
  • 但是mysql并不支持
create table test_user (
   id int,
   name varchar(20),
   sex varchar(1),
   check (sex ='男' or sex='女')
);

表的设计

一对一

  • 一对一设计表就比如学生表和账户表,一个账户对应到一个学生,一个学生也只有一个账户
  • 表示方法
    1.可以把这两个实体用一张表来表示
    2.可以用两张表来表示,其中一张表包含了另一个表的id

一对多

  • 一个学生应该处于一个班级中,一个班级可以包含多个学生
  • 表示方法:
    1.在班级表中,新增一列,表示这个班级里的学生id都有啥(mysql没有数组类型,redis可以)
    2.班级表不变,学生表中,新增一列classId

多对多

  • 多对多设计表就好比学生表和课程表,一个学生可以选多个课程,一个课程也可以被多个学生选择
  • 表示方法 :
    使用一个关联表,来表示两个实体之间的关系

多对多建表实例

-- 学生表
mysql> create table test_student (
    -> id int primary key,
    -> name varchar(10) default 'unknow'
    -> );
Query OK, 0 rows affected (0.03 sec)
-- 选课表
mysql> create table test_course (
    -> id int primary key,
    -> name varchar(20) default 'unknow'
    -> );
Query OK, 0 rows affected (0.02 sec)
-- 成绩表
mysql> create table test_score (
    -> studentId int,
    -> courseId int,
    -> score int,
    -> foreign key (studentId) references test_student(id),
    -> foreign key (courseId) references test_course(id)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> desc test_student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(10) | YES  |     | unknow  |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> desc test_coures;
ERROR 1146 (42S02): Table 'java_5_27.test_coures' doesn't exist
mysql> desc test_course;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | unknow  |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> desc test_score;
+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| studentId | int(11) | YES  | MUL | NULL    |       |
| courseId  | int(11) | YES  | MUL | NULL    |       |
| score     | int(11) | YES  |     | NULL    |       |
+-----------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)

插入数据到实例实现多对多

mysql> insert into test_student values (1, 'listen');
Query OK, 1 row affected (0.01 sec)

mysql> insert into test_course values (1, '数学');
Query OK, 1 row affected (0.00 sec)

mysql> insert into test_student values (2, 'Faker');
Query OK, 1 row affected (0.00 sec)

mysql> insert into test_course values (2, '数学');
Query OK, 1 row affected (0.00 sec)

mysql> insert into test_score values(1, 1, 90);
Query OK, 1 row affected (0.00 sec)

mysql> insert into test_score values (1, 2, 99);
Query OK, 1 row affected (0.00 sec)

mysql> insert into test_score values (2, 1, 50);
Query OK, 1 row affected (0.00 sec)

mysql> insert into test_score values (2, 2, 60);
Query OK, 1 row affected (0.00 sec)

mysql> select * from test_student;
+----+--------+
| id | name   |
+----+--------+
|  1 | listen |
|  2 | Faker  |
+----+--------+
2 rows in set (0.00 sec)

mysql> select * from test_course;
+----+--------+
| id | name   |
+----+--------+
|  1 | 数学   |
|  2 | 语文   |
+----+--------+
2 rows in set (0.00 sec)

mysql> select * from test_score;
+-----------+----------+-------+
| studentId | courseId | score |
+-----------+----------+-------+
|         1 |        1 |    90 |
|         1 |        2 |    99 |
|         2 |        1 |    50 |
|         2 |        2 |    60 |
+-----------+----------+-------+
4 rows in set (0.00 sec)

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版权声明:本文为CSDN博主「浦上青天」的原创文章

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