数据库（二）：初识sql语句

进击のpython

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数据库——初识sql语句

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前面提到了说，数据库管理系统就像我们曾经做过的输入命令返回结果的socket通信差不多

那既然提到了命令，在MySQL中，有一些基本的语句，就应该被知道

刚开始学的时候可能觉得这些这么多的英语单词，我记不住怎么办？

其实你会发现，好多的语句其实跟我们说话的方式很像（我会在介绍语句的时候说），记忆起来就不是很困难

如果是单词的问题，就只能建议你多敲多联系了，“无他，唯手熟尔”

数据库本质也就只是个文件对吧，那是个文件就有：增 查 改 删

所以我们会针对库，表，记录的这四个方法来进行介绍，一定注意，多敲！

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库（操作文件夹）

增

create database db1 charset utf8;

（创建 数据库 db1 编码设置 utf8）

mysql> create database db1 charset utf8;
Query OK, 1 row affected (0.05 sec)

这样就相当于建一个文件夹，其实也真的建立了一个文件夹

现在我们是在本机的数据库操作，所以数据都存放在了本地，那我们看看我们新建的数据库文件

找到你的数据库安装目录，能看到一个叫做data的文件夹，看名字也知道这个名字是数据的意思

我们打开它，是不是发现了我们新建的库：db1，而且是文件夹的形式吧！

数据库的命名也是有规范的：

1. 可以由字母、数字、下划线、＠、＃、＄
2. 区分大小写
3. 唯一性
4. 不能使用关键字如 create select
5. 不能单独使用数字
6. 最长128位

表也可以复制

复制表结构＋记录 （key不会复制: 主键、外键和索引）

create table new_service select * from t1;

mysql> create table new_service select * from t1;
Query OK, 2 rows affected (1.07 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> show
    -> tables;
+---------------+
| Tables_in_db1 |
+---------------+
| new_service   |
| t1            |
+---------------+
2 rows in set (0.00 sec)

mysql> desc new_service;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.01 sec)

mysql> select * from new_service;
+------+------+------+------+-------+------+
| num  | id   | name | sex  | hobby | job  |
+------+------+------+------+-------+------+
| NULL |    1 | 1    | 1    | 男    | 无   |
| NULL |    1 | 1    | 1    | 男    | 无   |
+------+------+------+------+-------+------+
2 rows in set (0.00 sec)

只复制表结构

create table t2 select * from t1 where 1=2; //条件为假，查不到任何记录

mysql> create table t2 select * from t1 where 1=2;
Query OK, 0 rows affected (1.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| new_service   |
| t1            |
| t2            |
+---------------+
3 rows in set (0.00 sec)

mysql> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.01 sec)

mysql> select * from t2;
Empty set (0.00 sec)

还有种简单的：

create table t3 like t1;

mysql> create table t3 like t1;
Query OK, 0 rows affected (1.03 sec)

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| new_service   |
| t1            |
| t2            |
| t3            |
+---------------+
4 rows in set (0.00 sec)

mysql> desc t3;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.01 sec)

mysql> select * from t3;
Empty set (0.00 sec)

查

show create database db1;

(展示 创建 数据库 db1)

mysql> show create database db1;
+----------+--------------------------------------------------------------+
| Database | Create Database                                              |
+----------+--------------------------------------------------------------+
| db1      | CREATE DATABASE `db1` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+--------------------------------------------------------------+
1 row in set (0.00 sec)

show databases;

(展示 所有数据库)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| db1                |
| mysql              |
| performance_schema |
| test               |
+--------------------+
6 rows in set (0.00 sec)

改

改其实无非就是改编码格式

alter database db1 charset gbk;

（修改 数据库 db1 编码格式 gbk）

mysql> alter database db1 charset gbk;
Query OK, 1 row affected (0.34 sec)

mysql> show create database db1;
+----------+-------------------------------------------------------------+
| Database | Create Database                                             |
+----------+-------------------------------------------------------------+
| db1      | CREATE DATABASE `db1` /*!40100 DEFAULT CHARACTER SET gbk */ |
+----------+-------------------------------------------------------------+
1 row in set (0.00 sec)

是不是编码格式被修改了！

删

drop database db1;

(删掉 数据库 db1)

mysql> drop database db1;
Query OK, 0 rows affected (0.41 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| test               |
+--------------------+
5 rows in set (0.00 sec)

这回所有的数据库中就不再有db1了

那你猜猜在data文件夹里还有db1这个文件夹了吗？？

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表（文件）

首先表（文件）必须要在库（文件夹）下，所以操作之前先建一个库db1（文件夹）

然后切换到这个文件夹下：

use db1;

（使用 db1）

mysql> use db1;
Database changed

其实这里可以提一句，你可以通过select database();来看你目前在哪个数据库下（文件夹里）

mysql> select database();
+------------+
| database() |
+------------+
| db1        |
+------------+
1 row in set (0.00 sec)

增

要创建一个表，是不是应该有相应字段（比如前一节例子的 姓名，性别之类的）,同时要给每一个字段的类型

create table t1(id int,name char,age int,sex char,hobby char,job char);

(创建 表 t1(id int型，name char型，... ...))

mysql> create table t1(id int,name char,age int,sex char,hobby char,job char);
Query OK, 0 rows affected (1.17 sec)

那我们去找一下db1文件夹，看看里面是不是新建了表（自己去看）

那总结起来，其实就是如下的格式：

create table 表名(
字段名1 类型[(宽度) 约束条件],
字段名2 类型[(宽度) 约束条件],
字段名3 类型[(宽度) 约束条件]
);

注意：

1. 在同一张表中，字段名是不能相同
2. 宽度和约束条件可选
3. 字段名和类型是必须的
4. 最后一个数据之后不要加'',''

aletr table t1 add times char;

(修改 表 t1 加 times 字符串类型)

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
5 rows in set (0.01 sec)

mysql> alter table t1
    -> add times char;
Query OK, 0 rows affected (1.29 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
| times | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.01 sec)

默认添加在最后,当然你也可以添加到开头

aletr table t1 add times char first;

mysql> alter table t1
    -> add num char first;
Query OK, 0 rows affected (1.45 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
| times | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
7 rows in set (0.01 sec)

查

show create table t1;

（展示 创建 表 t1）

mysql> show create table t1;
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                  |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) DEFAULT NULL,
  `name` char(1) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `sex` char(1) DEFAULT NULL,
  `hobby` char(1) DEFAULT NULL,
  `job` char(1) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

可能是会拿到这种乱七八糟的，就是因为他想给你拼个表格出来，但是太多了，就拼错了

但是关键的信息我们还是能看到的对吧

show tables;

(展示 所有表)

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t1            |
+---------------+
1 row in set (0.00 sec)

desc t1;

(查看列和数据类型 t1)

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
| times | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.35 sec)

看着舒服点了吧~~~

改

alter table t1 rename t2;

(修改 表 t1 重命名 t2)

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t1            |
+---------------+
1 row in set (0.06 sec)

mysql> alter table t1
    -> rename t2;
Query OK, 0 rows affected (0.57 sec)

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t2            |
+---------------+
1 row in set (0.00 sec)

alter table t1 modify name char(5);

（修改 表 t1 修改 name字段 char（5））

mysql> alter table t1 modify name char(5);
Query OK, 0 rows affected (1.57 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(5) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
| times | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.01 sec)

除了改字符长度，还可以修改字段

alter table t1 change name NAME char(5);

(修改 表 t1 改变 name 为 NAME char(5))

mysql> alter table t1 change name NAME char(5);
Query OK, 0 rows affected (0.51 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| NAME  | char(5) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
| times | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.01 sec)

删

drop table t1;

（删掉 表 t1）

mysql> drop table t1;
Query OK, 0 rows affected (0.55 sec)

mysql> show tables;
Empty set (0.00 sec)

看看你的data文件夹下的db1还有文件了吗？

alter table t1 drop times;

(修改 表 t1 删掉 times)

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
| times | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
7 rows in set (0.01 sec)

mysql> alter table t1
    -> drop times;
Query OK, 0 rows affected (1.48 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| num   | char(1) | YES  |     | NULL    |       |
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
6 rows in set (0.01 sec)

接下来的这些写法,你现在可能不知道什么意思,但是要记下来

1.修改存储引擎

mysql> alter table t1
他
    -> engine=innodb;

2.添加stu_num到name字段之后

mysql> alter table t1
    -> add stu_num int after name;

3.修改字段类型

mysql> alter table t1
    -> modify age int(3);

4.增加约束（针对已有的主键增加auto_increment）

mysql> alter table t1 modify id int(11) not null primary key auto_increment;
ERROR 1068 (42000): Multiple primary key defined
mysql> alter table t1 modify id int(11) not null auto_increment;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

5.对已经存在的表增加复合主键

mysql> alter table t1
    -> add primary key(host_ip,port);   

6.增加主键

mysql> alter table t1
    -> modify name char primary key;

7.增加主键和自动增长

mysql> alter table t1
    -> modify id int primary key auto_increment;

8.删除主键

mysql> alter table t1 modify id int(11) not null;  #删除自增约束

mysql> alter table t1                                 
    -> drop primary key; #删除主键

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记录

对记录的操作首先要有库和表对吧

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(1) | YES  |     | NULL    |       |
| sex   | char(1) | YES  |     | NULL    |       |
| hobby | char(1) | YES  |     | NULL    |       |
| job   | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
5 rows in set (0.44 sec)

增

insert t1(id,name,sex,hobby,job)values(1,'张三',18,'男','无业');

(插入 t1... ...)

mysql> insert t1(id,name,sex,hobby,job)values(1,'张三',18,'男','无业');
Query OK, 1 row affected, 3 warnings (0.44 sec)

多个添加：

insert t1(id,name,sex,hobby,job)values(1,'张三',18,'男','无业'),(2,'李四',20,'女','学生');

mysql> insert t1(id,name,sex,hobby,job)values(1,'张三',18,'男','无业'),(2,'李四',20,'女','学生');
Query OK, 2 rows affected, 6 warnings (0.38 sec)
Records: 2  Duplicates: 0  Warnings: 6

mysql> select * from t1;
+------+------+------+-------+------+
| id   | name | sex  | hobby | job  |
+------+------+------+-------+------+
|    1 | 1    | 1    | 男    | 无   |
|    1 | 张   | 1    | 男    | 无   |
|    2 | 李   | 2    | 女    | 学   |
+------+------+------+-------+------+
3 rows in set (0.00 sec)

查

select * from t1;

(查找 所有 从 t1)

mysql> select * from t1;
+------+------+------+-------+------+
| id   | name | sex  | hobby | job  |
+------+------+------+-------+------+
|    1 | 张   | 1    | 男    | 无   |
+------+------+------+-------+------+
1 row in set (0.00 sec)

这么写的前提是你已经在db1下面了，如果不在 后面的t1 就改写成db1.t1就好了

如果我只查姓名，就把*改成字段name 如果查姓名年龄，就改成 name，age

然后你就会发现，少字了

其实到这就该跟你说，char（数字）代表着这个字段容纳的字符个数，因为都是1，所以只留一个

我们可以修改一下！

改

update db1.t1 set name = '1';

(更新 db1里面的t1表 设置 name 为 1)

一旦是这么写的会发生什么呢？

mysql> update db1.t1 set name = '1';
Query OK, 2 rows affected (0.38 sec)
Rows matched: 3  Changed: 2  Warnings: 0

mysql> select * from t1;
+------+------+------+-------+------+
| id   | name | sex  | hobby | job  |
+------+------+------+-------+------+
|    1 | 1    | 1    | 男    | 无   |
|    1 | 1    | 1    | 男    | 无   |
|    2 | 1    | 2    | 女    | 学   |
+------+------+------+-------+------+
3 rows in set (0.00 sec)

我会发现我把所有的name都改掉了，那我要是只想改id=2的name呢？

update db1.t1 set name = '1' where id = 2;

mysql> update db1.t1 set name = 'haha' where id = 2;
Query OK, 1 row affected, 1 warning (0.38 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> select * from t1;
+------+------+------+-------+------+
| id   | name | sex  | hobby | job  |
+------+------+------+-------+------+
|    1 | 1    | 1    | 男    | 无   |
|    1 | 1    | 1    | 男    | 无   |
|    2 | h    | 2    | 女    | 学   |
+------+------+------+-------+------+
3 rows in set (0.00 sec)

删

delete from t1 where id = 2;

(删除 从t1 ... ...)

mysql> delete from t1 where id = 2;
Query OK, 1 row affected (0.38 sec)

mysql> select * from t1;
+------+------+------+-------+------+
| id   | name | sex  | hobby | job  |
+------+------+------+-------+------+
|    1 | 1    | 1    | 男    | 无   |
|    1 | 1    | 1    | 男    | 无   |
+------+------+------+-------+------+
2 rows in set (0.00 sec)

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***** *****