数据库基础(1)

-- 创建数据库

create database firstdb;

-- 使用数据库

use firstdb;

-- 创建数据表

create table pet(
name varchar(20),
owner varchar(20),
species varchar(20),
sex varchar(4),
birth DATE,
death DATE
);

-- 插入数据

insert into pet
VALUES('旺财','周星星','狗','公','1998-08-20',NULL);
insert into pet
VALUES('旺旺','周小星','狗','母','1997-08-20',NULL);
insert into pet
VALUES('汪汪','周大星','狗','公','1996-08-20',NULL);
insert into pet
VALUES('望望','星星周','狗','母','1995-08-20',NULL);
insert into pet
VALUES('大黄','周星驰','狗','公','1994-08-20',NULL);

--查看数据

select * from pet;

-- 删除数据

delete from pet where name='望望';

-- 修改数据

update pet set name='旺旺财' where name='旺财';

主键约束 primary key

-- 它能确定表中唯一的数据;使得数据不重复且不为空

mysql> create table user(
    -> id int primary key,
    -> name varchar(20));

插入一条id为1的张三

INSERT INTO user VALUES(1,'张三');

mysql> select * from user;
+----+--------+
| id | name   |
+----+--------+
|  1 | 张三   |
+----+--------+

--再插入一条id为1的张三就会报错

mysql> INSERT INTO user VALUES(2,'张三');
ERROR 1062 (23000): Duplicate entry '1' for key 'user.PRIMARY'

--插入一条id为2的张三则不会报错

mysql> INSERT INTO user VALUES(2,'张三');
Query OK, 1 row affected (0.06 sec)

mysql> select * from user;
+----+--------+
| id | name   |
+----+--------+
|  1 | 张三   |
|  2 | 张三   |
+----+--------+
2 rows in set (0.00 sec)

-- 主键为null则报错

mysql> INSERT INTO user VALUES(NULL,'张三');
ERROR 1048 (23000): Column 'id' cannot be null

联合主键

CREATE TABLE user2(
    id int,
    name varchar(20),
    password varchar(20),
    primary key(id,name)
);

-- 只要两个主键加起来不相等,则不会报错

mysql> INSERT INTO user2 VALUES(1,'张三','123');
Query OK, 1 row affected (0.05 sec)
mysql> INSERT INTO user2 VALUES(2,'张三','123');
Query OK, 1 row affected (0.05 sec)

mysql> select * from user2;
+----+--------+----------+
| id | name   | password |
+----+--------+----------+
|  1 | 张三   | 123      |
|  2 | 张三   | 123      |
+----+--------+----------+

-- 只要任一主键为空,则会报错

mysql> INSERT INTO user2 VALUES(null,'李四','123');
ERROR 1048 (23000): Column 'id' cannot be null

自增约束 auto_increment

-- 与主键搭配使用 插入数据时不插入主键数据,系统会自动为你添加一个值

CREATE TABLE user3(
    id int primary key AUTO_INCREMENT,
    name varchar(20)
);

-- 第一次插入

mysql> INSERT INTO user3 (name) VALUES('张三');
Query OK, 1 row affected (0.12 sec)

mysql> select * from user3;
+----+--------+
| id | name   |
+----+--------+
|  1 | 张三   |
+----+--------+
1 row in set (0.00 sec)

-- 第二次插入

mysql> INSERT INTO user3 (name) VALUES('张三');
Query OK, 1 row affected (0.07 sec)

mysql> select * from user3;
+----+--------+
| id | name   |
+----+--------+
|  1 | 张三   |
|  2 | 张三   |
+----+--------+

插入主键 (不常见)

CREATE TABLE user4(
    id int,
    name varchar(20)
);

mysql> ALTER TABLE user4 add primary key(id);   
Query OK, 0 rows affected (0.67 sec)
Records: 0  Duplicates: 0  Warnings: 0

--使用modify也可以添加主键 (modify修改数值类型)
--ALTER TABLE user4 MODIFY id int primary key;

mysql> DESC user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

删除主键(不常见)

mysql> ALTER TABLE user4 drop primary key;
Query OK, 0 rows affected (0.67 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> DESC user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

唯一约束 unique

-- primary key = unique + not null
-- unique 使用方法和 primary key 几乎相同(删除的方法不相同),但是使用情景不同.
-- unique 可定义多个,primary key 原则上只定义一个.

CREATE TABLE user5(
    id int,
    name varchar(20) unique
);

mysql> desc user5;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

-- 删除唯一约束                注意↓↓
mysql> ALTER TABLE user5 drop index name;
Query OK, 0 rows affected (0.37 sec)
Records: 0  Duplicates: 0  Warnings: 0


mysql> desc user5;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

-- 添加唯一约束               注意↓↓
mysql> alter table user5 add unique(name);
Query OK, 0 rows affected (0.60 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user5;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

非空约束 not null

-- 即不允许为空,约束后,约束的值插入为空则报错

CREATE TABLE user6(
    id int,
    name varchar(20) not null
);

mysql> desc user6;
                      --↓↓↓NULL中YES为允许,NO为不允许
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> INSERT INTO user6 (id) VALUES('1');
ERROR 1364 (HY000): Field 'name' does not have a default value;

默认约束 default

-- 当你未给 添加了默认约束的值 添加值时,系统会给你添加设置的默认值

CREATE TABLE user7(
    id int,
    name varchar(20) default '无名',
    age varchar(10)
);
mysql>desc user7;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | 无名    |       |
| age   | varchar(10) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> INSERT INTO user7 (id, age) VALUES(1,18);
Query OK, 1 row affected (0.08 sec)

mysql> SELECT * FROM user7;
+------+--------+------+
| id   | name   | age  |
+------+--------+------+
|    1 | 无名   | 18   |
+------+--------+------+
1 row in set (0.00 sec)

外键约束

-- 父表与子表相联系,在子表中约束,子表中 约束后的值 只能使用 父表中有的值

CREATE TABLE classes(
    id int primary key,
    name varchar(20)
);

CREATE TABLE students(
    id int primary key,
    name varchar(20),
    class_id int,
    foreign key(class_id) references classes(id)  --该表的class_id连接了classes表中的id值
);


mysql> desc students;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int         | NO   | PRI | NULL    |       |
| name     | varchar(20) | YES  |     | NULL    |       |
| class_id | int         | YES  | MUL | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

INSERT INTO classes VALUES(1,'一班');
INSERT INTO classes VALUES(2,'二班');
INSERT INTO classes VALUES(3,'三班');
INSERT INTO classes VALUES(4,'四班');

mysql> select * from classes;
+----+--------+
| id | name   |
+----+--------+
|  1 | 一班   |
|  2 | 二班   |
|  3 | 三班   |
|  4 | 四班   |
+----+--------+
4 rows in set (0.00 sec)

INSERT INTO students VALUES(1001,'张三',1);
INSERT INTO students VALUES(2001,'张三',2);
INSERT INTO students VALUES(3001,'张三',3);
INSERT INTO students VALUES(4001,'张三',4);

mysql> select * from students;
+------+--------+----------+
| id   | name   | class_id |
+------+--------+----------+
| 1001 | 张三   |        1 |
| 2001 | 张三   |        2 |
| 3001 | 张三   |        3 |
| 4001 | 张三   |        4 |
+------+--------+----------+
4 rows in set (0.00 sec)

-- students的class_id插入了一个父表中id不存在的值→报错

mysql> INSERT INTO students VALUES(5001,'张三',5);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`firstdb`.`students`, CONSTRAINT `students_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))

-- 父表被绑定后 无法删除已经被使用的值

mysql> delete from classes where id=1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`firstdb`.`students`, CONSTRAINT `students_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))