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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