数据库约束
not null
unique
default
primary key 主键
foreign key 外键
check
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
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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