MySQL~数据库约束(null、unique、primary key、default、foreign key、check、auto_increment) 与表的设计(一对一、一对多、多对多)

数据库约束

not null

• 指定某列的存储不能为null值

unique

• 保证某列的每行必须有唯一的值

default

• 规定给列赋值时的默认值

primary key 主键

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

foreign key 外键

• 外键约束,保证一个表中的数据匹配另一个表中的值参照完整性.
• 一般习惯放到最后
• 外键约束会影响表的删除,例如下面实例的class表被关联,所以他不能被轻易删除

数据库约束实例

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(了解)

• 作为了解,类似java中的枚举,保证列中的值符合指定的条件
• MySQL使用时不报错，但忽略该约束：

create table test_user (
   id int,
   name varchar(20),
   sex varchar(1),
   check (sex ='男' or sex='女')
);

auto_increment 自增

• 自增特点:

1. 如果表中没有记录,自增从一开始
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曾

mysql> create table test (
    -> id int primary key auto_increment,
    -> name varchar(20)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> insert into test values (null, 'a');
Query OK, 1 row affected (0.01 sec)

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

mysql> delete from test where id = 1;
Query OK, 1 row affected (0.01 sec)

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

mysql> select * from test;
+----+------+
| id | name |
+----+------+
|  2 | a    |
+----+------+
1 row in set (0.00 sec)

表的设计

一对一

• 一对一设计表就比如人的表与身份证号的表,一个人只能有一个身份证号码,一个身份证号码也只对于着一个表

一对多

• 一对多设计表,就是我上述设计的实例那俩个表,其一个学生只能对应一个班级,但是一个班级可以对应多个学生

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

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)