MySQL数据库约束，表的设计

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系列专栏：MySQL通关系列

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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