INSERT INTO user2 VALUES (1,'老李','123456');
SELECT * FROM user2;
+----+------+----------+
| id | name | password |
+----+------+----------+
| 1 | 老李 | 123456 |
| 1 | 老王 | 123456 |
| 2 | 老王 | 123456 |
+----+------+----------+
场景:表中有班级号以及学生座位号,我们可以用班级号+学生的座位号可以准确的定位一个学生,如:(1班5号可以准确的确定一个学生)
3.自增约束:
CREATE TABLE user3(
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20)
);
运行DESCRIBE user3;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
INSERT INTO user3(name) VALUES('张三');
INSERT INTO user3(name) VALUES('李四');
+----+------+
| id | name |
+----+------+
| 1 | 张三 |
| 2 | 李四 |
+----+------+
没有自定义id值 但是自动生成了id,这个ID的增长不会以表里面的数据来决定，而是每次插入一条数据增加一个，删除数据不会更改这个值。
4.唯一约束:
CREATE TABLE user5(
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20)
);
运行 DESCRIBE user5;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
新增name为唯一约束:
ALTER TABLE user5 ADD UNIQUE(name);
运行 DESCRIBE user5;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+----------------+
测试:插入数据
INSERT INTO user5(name) VALUES ('cc');
运行 SELECT * FROM user5; 查看结果:
+----+------+
| id | name |
+----+------+
| 1 | cc |
+----+------+
再次插入INSERT INTO user5(name) VALUES ('cc');
出现:ERROR 1062 (23000): Duplicate entry 'cc' for key 'name'
换个试试 INSERT INTO user5(name) VALUES ('aa');
运行 SELECT * FROM user5; 查看结果:
+----+------+
| id | name |
+----+------+
| 3 | aa |
| 1 | cc |
+----+------+
总结一下:
主键约束(primary key)中包含了唯一约束
场景:业务需求:设计一张用户注册表,用户姓名必须要用手机号来注册,而且手机号和用户名称都不能为空,那么:
CREATE TABLE user_test(
id INT PRIMARY KEY AUTO_INCREMENT COMMENT'主键id',
name VARCHAR(20) NOT NULL COMMENT'用户姓名,不能为空',
phone_number VARCHAR(20) UNIQUE NOT NULL COMMENT'用户手机,不能重复且不能为空'
);
运行 DESCRIBE user_test;
+--------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| phone_number | int(11) | NO | UNI | NULL | |
+--------------+-------------+------+-----+---------+----------------+
这样的话就达到了每一个手机号都只能出现一次,达到了每个手机号只能被注册一次.
用户姓名可以重复,但是手机号码却不能重复,复合正常的逻辑需求
5.非空约束:
在上面的蓝字中已经添加了非空约束: NOT NULL;
name和phone_number都设置了非空,先只设置name参数不设置phone_number参数试一试
INSERT INTO user_test (name) VALUES ('张三');
会出现Field 'phone_number' doesn't have a default value
两个非空参数一起设置:
INSERT INTO user_test (name,phone_number) VALUES ('张三','12345678901');
+----+------+--------------+
| id | name | phone_number |
+----+------+--------------+
| 1 | 张三 | 12345678901 |
+----+------+--------------+
6.默认约束
CREATE TABLE user6(
id int PRIMARY KEY AUTO_INCREMENT COMMENT'主键id',
name VARCHAR(20) NOT NULL COMMENT'用户姓名不能为空',
phone_number VARCHAR(20) NOT NULL COMMENT'用户手机号,不能为空',
status INT DEFAULT 0 COMMENT'用户状态0:启用 1:禁封 默认:0'
);
运行DESCRIBE user6;
+--------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| phone_number | varchar(20) | NO | | NULL | |
| status | int(11) | YES | | 0 | |
+--------------+-------------+------+-----+---------+----------------+
插入数据:
INSERT INTO user6(name,phone_number) VALUES ('aa','123');
INSERT INTO user6(name,phone_number) VALUES('bb','1234');
INSERT INTO user6(name,phone_number) VALUES('cc','1263456');
查看数据:SELECT * FROM user6;
+----+------+--------------+--------+
| id | name | phone_number | status |
+----+------+--------------+--------+
| 1 | aa | 123 | 0 |
| 2 | bb | 1234 | 0 |
| 3 | cc | 1263456 | 0 |
+----+------+--------------+--------+
我们没有设置status的值,但是给我们创建了默认值 0.
应用场景:
业务需求:找正常的用户,对这些正常用户进行发放优惠卷或者积分之类的东西,而被禁封的用户我们不让其参加多动.
我们想要封用户只要将status的值从0改为1就行了,当然我们取用户的时候必须要先判断status是否是0.若是1.说明该用户已经被禁封.
先封手机号为'1234'的用户:
UPDATE user6 SET status = 1 WHERE phone_number= '1234';
SELECT * FROM user6;
+----+------+--------------+--------+
| id | name | phone_number | status |
+----+------+--------------+--------+
| 1 | aa | 123 | 0 |
| 2 | bb | 1234 | 1 |
| 3 | cc | 1263456 | 0 |
+----+------+--------------+--------+
status为1,说明用户已经被封,该用户不可以参加活动
我们取用户的时候加上status的判断,如:
SELECT * FROM user6 WHERE status = 0;
+----+------+--------------+--------+
| id | name | phone_number | status |
+----+------+--------------+--------+
| 1 | aa | 123 | 0 |
| 3 | cc | 1263456 | 0 |
+----+------+--------------+--------+
7.外键约束
CREATE TABLE classes(
id INT PRIMARY KEY AUTO_INCREMENT ,
name VARCHAR(20)
);
运行DESCRIBE classes;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
CREATE TABLE student(
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20),
class_id int,
FOREIGN KEY (class_id) REFERENCES classes(id)
);
//FOREIGN :外来 REFERENCES:应用,参考
运行DESCRIBE student;
+----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| class_id | int(11) | YES | MUL | NULL | |
+----------+-------------+------+-----+---------+----------------+
班级插入数据:
INSERT INTO CLASSES (name) VALUES ('一班');
INSERT INTO CLASSES (name) VALUES ('二班');
INSERT INTO CLASSES (name) VALUES ('三班');
INSERT INTO CLASSES (name) VALUES ('四班');
查看数据 SELECT * FROM classes;
+----+------+
| id | name |
+----+------+
| 1 | 一班 |
| 2 | 二班 |
| 3 | 三班 |
| 4 | 四班 |
+----+------+
学生插入数据:
INSERT INTO student (name,class_id) VALUES ('小赵',1);
INSERT INTO student (name,class_id) VALUES ('小钱',2);
INSERT INTO student (name,class_id) VALUES ('小孙',3);
INSERT INTO student (name,class_id) VALUES ('小李',4);
查看数据 SELECT * FROM student;
+----+------+----------+
| id | name | class_id |
+----+------+----------+
| 1 | 小赵 | 1 |
| 2 | 小钱 | 2 |
| 3 | 小孙 | 3 |
| 4 | 小李 | 4 |
+----+------+----------+
若是像插入班级为5的数据 如:
INSERT INTO student (name,class_id) VALUES ('小周',5);
报错: Cannot add or update a child row
我们删除正在被学生表引用的'四班'试试:
DELETE classes WHERE name = '四班';
出现:Cannot delete or update a parent row:不能删除主表中的行
我们先删除学生表中的 '小李'从而解除班级中'四班'的外键约束,再来删除'四班'(因为小李引用了四班)
DELETE FROM student WHERE name = '小李';
再次删除classes表中的'四班';
DELETE FROM classes WHERE name = '四班';
最后: SELECT * FROM classes;
+----+------+
| id | name |
+----+------+
| 1 | 一班 |
| 2 | 二班 |
| 3 | 三班 |
+----+------+
'四班'被成功删除!
总结:
1.主表中没有的数据,在附表中,是不可以使用的.
2.主表中记录的数据现在正在被附表所引用,那么主表中正在被引用的数据不可以被删除
3.若要想删除,先将附表中的数据删除在删除主表数据
4.对于外键约束大家可以联想 省,市 来进行联想 (市必须要依赖于省,只要省还有一个市在引用,那么就不可以删除省,要不然市就没有省了. 那么我们想删除省,必须要将该省下所有的市全部删除之后,才可以删除这个省)
8.如何建表之后添加主键约束
CREATE TABLE user4(
id INT,
name VARCHAR(20)
);
运行DESCRIBE user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
加入主键约束:
ALTER TABLE user4 add PRIMARY KEY(id);
再次运行DESCRIBE user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
删除主键约束:
ALERT TABLE user4 DROP PRIMARY KEY;
运行DESCRIBE user4查看表结构:
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
使用modify 修改字段.添加约束:
ALTER TABLE user4 MODIFY id INT PRIMARY key;
使用DESCRIBE user4 查看表结构:
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
给主键设置自增长:
ALTER TABLE user4 MODIFY id INT AUTO_INCREMENT;
运行 DESCRIBE user4 查看表结构:
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
