初识SQL语句
SQL（Structured Query Language ）
即结构化查询语言
SQL语言主要用于存取数据、查询数据、更新数据和管理关系数据库系统,SQL语言由IBM开发。SQL语言分为3种类型： DDL语句 数据库定义语言： 数据库、表、视图、索引、存储过程，例如CREATE DROP ALTER DML语句 数据库操纵语言： 插入数据INSERT、删除数据DELETE、更新数据UPDATE、查询数据SELECT DCL语句 数据库控制语言： 例如控制用户的访问权限GRANT、REVOKE
1.如果你在cmd中输入命令的时候，输入错了就用\c跳出
2.如果你在cmd中输入命令的时候，输入错了，且带有引号，就用引号\c跳出
3. \s查看配置信息
数据库操作
即操作文件夹
数据库命名规则：
可以由字母、数字、下划线、＠、＃、＄ 区分大小写 唯一性 不能使用关键字如 create select 不能单独使用数字 最长128位
数据库的操作
增：create database db1 charset utf8; 删：drop database db1; 改：alter database db1 charset gbk; 查： show databases; #查看所有的数据库 show create database db1; #查看db1数据库
举例
mysql> create database db1 charset utf8
-> ;
Query OK, 1 row affected (0.01 sec)
mysql> show create database db1;
+----------+--------------------------------------------------------------+
| Database | Create Database |
+----------+--------------------------------------------------------------+
| db1 | CREATE DATABASE `db1` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+--------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| db1 |
| mysql |
| performance_schema |
| test |
+--------------------+
5 rows in set (0.00 sec)
mysql> alter database db1 charset gbk;
Query OK, 1 row affected (0.00 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)
mysql> drop database db1;
Query OK, 0 rows affected (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| test |
+--------------------+
4 rows in set (0.00 sec)
表的操作
即操作文件
基础操作
切换到数据库（文件夹下）:use db1
增：create table t1(id int,name char(10)) engine=innodb;
删：drop table t1;
改：alter table t1 add age int;
alter table t1 modify name char(12);
查：show tables; #查看所有表
show create table t1; #查看t1表
desc t1;#查看表结构
show create table t1\G; #查看表详细结构，可加\G
select * from t1; #查看所有的表数据
举例
mysql> use db1;
Database changed
mysql> create table t2(id int,name char(10)) engine=innodb charset=utf
Query OK, 0 rows affected (0.03 sec)
mysql> show create table t2;
+-------+-------------------------------------------------------------
-----------------------------------------------+
| Table | Create Table
|
+-------+-------------------------------------------------------------
-----------------------------------------------+
| t2 | CREATE TABLE `t2` (
`id` int(11) DEFAULT NULL,
`name` char(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-------------------------------------------------------------
-----------------------------------------------+
1 row in set (0.00 sec)
mysql> show create table t2\G;
*************************** 1. row ***************************
Table: t2
Create Table: CREATE TABLE `t2` (
`id` int(11) DEFAULT NULL,
`name` char(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
ERROR:
No query specified
mysql>
mysql> desc t2;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(10) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> alter table t2 modify name char(20);
Query OK, 0 rows affected (0.07 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t2;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> alter table t2 add age int;
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table t2\G;
*************************** 1. row ***************************
Table: t2
Create Table: CREATE TABLE `t2` (
`id` int(11) DEFAULT NULL,
`name` char(20) DEFAULT NULL,
`age` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
ERROR:
No query specified
mysql> select * from t2;
Empty set (0.00 sec)
复制表与删除表
show tables; #查看库中所有表 create table t5 like t2; #只复制表结构，包括key，index等 create table t5 select * from t2 where 1=2; #只复制表结构，不包括key，index等 create table t4 select * from t2; #复制表结构和数据，不包括key，index等 drop table t4; 删除表
举例
mysql> create table t4 select * from t3 where 1=2; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> mysql> create table t5 like t2; Query OK, 0 rows affected (0.01 sec) mysql> desc t5; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | char(20) | NO | PRI | | | | grade | int(11) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 3 rows in set (0.01 sec) mysql> mysql> drop table t3; Query OK, 0 rows affected (0.01 sec) mysql> show tables; +---------------+ | Tables_in_db1 | +---------------+ | t1 | | t2 | | t4 | | t5 | +---------------+ 4 rows in set (0.00 sec) mysql> mysql> mysql> insert into t2 values(1,'egon',99),(2,'alex',98); Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> select * from t2; +----+------+-------+ | id | name | grade | +----+------+-------+ | 1 | egon | 99 | | 2 | alex | 98 | +----+------+-------+ 2 rows in set (0.00 sec) mysql> drop table t4; Query OK, 0 rows affected (0.01 sec) mysql> drop table t5; Query OK, 0 rows affected (0.01 sec) mysql> mysql> create table t4 select * from t2; Query OK, 2 rows affected (0.04 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> desc t2; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | char(20) | NO | PRI | | | | grade | int(11) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 3 rows in set (0.01 sec) mysql> desc t4; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | | name | char(20) | NO | | | | | grade | int(11) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 3 rows in set (0.01 sec) mysql> select * from t4; +----+------+-------+ | id | name | grade | +----+------+-------+ | 1 | egon | 99 | | 2 | alex | 98 | +----+------+-------+ 2 rows in set (0.00 sec) mysql> create table t5 select * from t2 where 1=2; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> select * from t5; Empty set (0.00 sec) mysql> mysql> create table t6 like t2; Query OK, 0 rows affected (0.01 sec) mysql> desc t6; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | char(20) | NO | PRI | | | | grade | int(11) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 3 rows in set (0.01 sec) mysql> select * from t6; Empty set (0.00 sec) mysql> show tables; +---------------+ | Tables_in_db1 | +---------------+ | t1 | | t2 | | t4 | | t5 | | t6 | +---------------+ 5 rows in set (0.00 sec) mysql> drop table t5; Query OK, 0 rows affected (0.01 sec) mysql> drop table t4; Query OK, 0 rows affected (0.01 sec) mysql> show tables; +---------------+ | Tables_in_db1 | +---------------+ | t1 | | t2 | | t6 | +---------------+ 3 rows in set (0.00 sec)
操作文件中的内容/记录
增：insert into db1.t2 values(1,'egon',18),(2,'alex',20),(3,'eva',21); #如果t2不给参数，默认按照位置参数依次传参
删：delete from t2 where id = 2;
#对于清空记录有两种方式，但是推荐后者
delete from t2;
truncate t2; #当数据量比较大的情况下，使用这种方式，删除速度快
改：update t2 set name = 'ww' where id=3;
update t2 set name= 'ww' where name = 'alex';
alter table t2 modify id int primary key auto_increment; 修改id为主键并且自增
查：select * from t2; #查看t2里所有的数据
select name from t2; #查看t2里所有的name
select id,name from t2; #查看t2里所有的id,name
添加字段：alter table t2 add grade int;
修改存储引擎 ：alter table t2 engine=myisam;
删除字段: alter table t2 drop age;
删除自增约束: alter table t2 modify id int not null;
删除主键: alter table t2 drop primary key;
增加主键:alter table t2 modify name varchar(10) not null primary key;
增加复合主键：alter table t2 add primary key(id,name);
举例
mysql> insert into db1.t2 values(1,'egon',18),(2,'alex',20),(3,'eva',21);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from t2\G;
*************************** 1. row ***************************
id: 1
name: egon
age: 18
*************************** 2. row ***************************
id: 2
name: alex
age: 20
*************************** 3. row ***************************
id: 3
name: eva
age: 21
3 rows in set (0.00 sec)
mysql> select * from t2;
+------+------+------+
| id | name | age |
+------+------+------+
| 1 | egon | 18 |
| 2 | alex | 20 |
| 3 | eva | 21 |
+------+------+------+
3 rows in set (0.00 sec)
mysql> alter table t2 modify id int primary key auto_increment;
Query OK, 3 rows affected (0.08 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> show create table t2\G;
*************************** 1. row ***************************
Table: t2
Create Table: CREATE TABLE `t2` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` char(20) DEFAULT NULL,
`age` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
mysql> select id,name from t2;
+----+------+
| id | name |
+----+------+
| 1 | wy |
| 2 | ww |
| 3 | eva |
+----+------+
3 rows in set (0.00 sec)
mysql> delete from t2 where id=1;
Query OK, 1 row affected (0.00 sec)
mysql> select id,name from t2;
+----+------+
| id | name |
+----+------+
| 2 | ww |
| 3 | eva |
+----+------+
2 rows in set (0.00 sec)
mysql> truncate table t2;
Query OK, 0 rows affected (0.03 sec)
mysql> select id,name from t2;
Empty set (0.00 sec)
mysql> alter table t2 drop age;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t2;
+-------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | char(20) | YES | | NULL | |
| grade | int(11) | YES | | NULL | |
+-------+----------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
mysql> alter table t2 add primary key(id,name);
ERROR 1068 (42000): Multiple primary key defined
mysql>
mysql> alter table t2 modify id int not null;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t2;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | char(20) | YES | | NULL | |
| grade | int(11) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
3 rows in set (0.01 sec)
mysql> alter table t2 drop primary key;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t2;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| name | char(20) | YES | | NULL | |
| grade | int(11) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
3 rows in set (0.01 sec)
mysql> alter table t2 add primary key(id,name);
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
查看当前用户和当前数据库
mysql> select user(); mysql> select database();
字符编码utf8和utf8mb4
一、简介
MySQL在5.5.3之后增加了这个utf8mb4的编码，mb4就是most bytes 4的意思，专门用来兼容四字节的unicode。好在utf8mb4是utf8的超集，除了将编码改为utf8mb4外不需要做其他转换。当然，为了节省空间，一般情况下使用utf8也就够了。
二、内容描述
那上面说了既然utf8能够存下大部分中文汉字,那为什么还要使用utf8mb4呢? 原来mysql支持的 utf8 编码最大字符长度为 3 字节，如果遇到 4 字节的宽字符就会插入异常了。三个字节的 UTF-8 最大能编码的 Unicode 字符是 0xffff，也就是 Unicode 中的基本多文种平面(BMP)。也就是说，任何不在基本多文本平面的 Unicode字符，都无法使用 Mysql 的 utf8 字符集存储。包括 Emoji 表情(Emoji 是一种特殊的 Unicode 编码，常见于 ios 和 android 手机上)，和很多不常用的汉字，以及任何新增的 Unicode 字符等等(utf8的缺点)。
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