doc: add mysql.md cheatsheet.

README.md

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docs/mysql.md

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+MySQL 备忘清单
+===
+
+本备忘单旨在快速理解 [MySQL](https://mysql.com) 所涉及的主要概念，提供了最常用的SQL语句，供您参考。
+
+
+入门
+---
+
+### 介绍
+<!--rehype:wrap-class=row-span-3-->
+
+MySQL 为关系型数据库(Relational Database Management System)，一个关系型数据库由一个或数个表格组成，如下所示的一个表格
+
+----
+
+```bash
+    name ▼ 键            ▼ 列(col)
+┌┈┈┈┈┬┈┈┈┈┈┈┈┈┬┈┈┈┈┈┈┬┈┈┈┈┈┈┈┐
+┆ id ┆ name   ┆ uid  ┆ level ┆  ◀ 表头header
+├┈┈┈┈┼┈┈┈┈┈┈┈┈┤┈┈┈┈┈┈┤┈┈┈┈┈┈┈┤
+┆  1 ┆ mysql  ┆ 0    ┆ 3     ┆
+├┈┈┈┈┼┈┈┈┈┈┈┈┈┤┈┈┈┈┈┈┤┈┈┈┈┈┈┈┤
+┆  2 ┆ redis  ┆ 12   ┆ 1     ┆  ◀ 行 row
+└┈┈┈┈┴┈┈┈┈┈┈┈┈┴┈┈┈┈┈┈┴┈┈┈┈┈┈┈┘
+    redis ▲ 值
+```
+
+----
+
+- `表头(header)` 每一列的名称
+- `列(col)` 具有相同数据类型的数据的集合
+- `行(row)` 每一行用来描述某个人/物的具体信息
+- `值(value)` 行的具体信息，每个值与该列数据类型相同
+- `键(key)` 用来识别某个特定的人/物的方法，有唯一性
+
+### 登录MySQL
+
+```shell
+# 默认用户名<root>，-p 是密码，
+# ⚠️参数后面不需要空格
+mysql -h 127.0.0.1 -u <用户名> -p<密码>
+mysql -D 数据库名 -h 主机名 -u 用户名 -p
+mysql -h <host> -P <端口号> -u <user> -p [db_name]
+mysql -h <host> -u <user> -p [db_name]
+```
+
+### 常用的
+<!--rehype:wrap-class=row-span-3-->
+
+#### 数据库 Database
+
+:-|:-
+:-|:-
+`CREATE DATABASE` db `;` | `创建`数据库
+`SHOW DATABASES;`        | `列出`数据库
+`USE` db`;`              | `切换`到数据库
+`CONNECT` db `;`         | `切换`到数据库
+`DROP DATABASE` db`;`    | `删除`数据库
+
+#### 表 Table
+
+:-|:-
+:-|:-
+`SHOW TABLES;`           | 列出当前数据库的表
+`SHOW FIELDS FROM` t`;`  | 表的列表字段
+`DESC` t`;`              | 显示表格结构
+`SHOW CREATE TABLE `t`;` | 显示创建表sql
+`TRUNCATE TABLE `t`;`    | 删除表中的所有数据
+`DROP TABLE `t`;`        | 删除表格
+
+#### Proccess
+
+:-|:-
+:-|:-
+`show processlist;` | 列出进程
+`kill` pid`;`       | 杀死进程
+
+### 查看 MySQL 信息
+
+```shell
+# 显示当前mysql的version的各种信息
+mysql> status;
+# 显示当前mysql的version信息
+mysql> select version(); 
+# 查看 MySQL 端口号
+mysql> show global variables like 'port';
+```
+
+### 退出MySQL会话
+
+```bash
+mysql> exit 
+```
+
+退出 `quit;` 或 `\q;` 一样的效果
+
+### 备份
+
+创建备份
+
+```sql
+mysqldump -u user -p db_name > db.sql
+```
+
+导出不带架构的数据库
+
+```shell
+mysqldump -u user -p db_name --no-data=true --add-drop-table=false > db.sql
+```
+<!--rehype:className=wrap-text -->
+
+恢复备份
+
+```shell
+mysql -u user -p db_name < db.sql
+```
+
+MySQL 示例
+------
+
+### 管理表格
+
+创建一个包含三列的新表
+
+```sql
+CREATE TABLE t (
+    id    INT,
+    name  VARCHAR DEFAULT NOT NULL,
+    price INT DEFAULT 0
+    PRIMARY KEY(id)
+);
+```
+
+从数据库中删除表
+
+```sql
+DROP TABLE t ;
+```
+
+向表中添加新列
+
+```sql
+ALTER TABLE t ADD column;
+```
+
+从表中删除列c
+
+```sql
+ALTER TABLE t DROP COLUMN c ;
+```
+
+添加约束
+
+```sql
+ALTER TABLE t ADD constraint;
+```
+
+删除约束
+
+```sql
+ALTER TABLE t DROP constraint;
+```
+
+将表从t1重命名为t2
+
+```sql
+ALTER TABLE t1 RENAME TO t2;
+```
+
+将列c1重命名为c2
+
+```sql
+ALTER TABLE t1 RENAME c1 TO c2 ;
+```
+
+删除表中的所有数据
+
+```sql
+TRUNCATE TABLE t;
+```
+
+### 从表中查询数据
+
+从表中查询列c1、c2中的数据
+
+```sql
+SELECT c1, c2 FROM t
+```
+
+查询表中的所有行和列
+
+```sql
+SELECT * FROM t
+```
+
+查询数据并使用条件筛选行
+
+```sql
+SELECT c1, c2 FROM t
+WHERE condition
+```
+
+查询表中的不同行
+
+```sql
+SELECT DISTINCT c1 FROM t
+WHERE condition
+```
+
+按升序或降序对结果集排序
+
+```sql
+SELECT c1, c2 FROM t
+ORDER BY c1 ASC [DESC]
+```
+
+跳过行的偏移并返回下n行
+
+```sql
+SELECT c1, c2 FROM t
+ORDER BY c1 
+LIMIT n OFFSET offset
+```
+
+使用聚合函数对行进行分组
+
+```sql
+SELECT c1, aggregate(c2)
+FROM t
+GROUP BY c1
+```
+
+使用HAVING子句筛选组
+
+```sql
+SELECT c1, aggregate(c2)
+FROM t
+GROUP BY c1
+HAVING condition
+```
+
+### 从多个表查询
+<!--rehype:wrap-class=row-span-2-->
+
+内部连接 t1 和 t2
+
+```sql
+SELECT c1, c2 
+FROM t1
+INNER JOIN t2 ON condition
+```
+
+左连接t1和t1
+
+```sql
+SELECT c1, c2 
+FROM t1
+LEFT JOIN t2 ON condition
+```
+
+右连接t1和t2
+
+```sql
+SELECT c1, c2 
+FROM t1
+RIGHT JOIN t2 ON condition
+```
+
+执行完全外部连接
+
+```sql
+SELECT c1, c2 
+FROM t1
+FULL OUTER JOIN t2 ON condition
+```
+
+生成表中行的笛卡尔积
+
+```sql
+SELECT c1, c2 
+FROM t1
+CROSS JOIN t2
+```
+
+执行交叉连接的另一种方法
+
+```sql
+SELECT c1, c2 
+FROM t1, t2
+```
+
+使用INNER Join子句将t1连接到自身
+
+```sql
+SELECT c1, c2
+FROM t1 A
+INNER JOIN t1 B ON condition
+```
+
+使用SQL运算符，合并两个查询中的行
+
+```sql
+SELECT c1, c2 FROM t1
+UNION [ALL]
+SELECT c1, c2 FROM t2
+```
+
+返回两个查询的交集
+
+```sql
+SELECT c1, c2 FROM t1
+INTERSECT
+SELECT c1, c2 FROM t2
+```
+
+从另一个结果集中减去一个结果集
+
+```sql
+SELECT c1, c2 FROM t1
+MINUS
+SELECT c1, c2 FROM t2
+```
+
+使用模式匹配%查询行_
+
+```sql
+SELECT c1, c2 FROM t1
+WHERE c1 [NOT] LIKE pattern
+```
+
+查询列表中的行
+
+```sql
+SELECT c1, c2 FROM t
+WHERE c1 [NOT] IN value_list
+```
+
+查询两个值之间的行
+
+```sql
+SELECT c1, c2 FROM t
+WHERE  c1 BETWEEN low AND high
+```
+
+检查表中的值是否为NULL
+
+```sql
+SELECT c1, c2 FROM t
+WHERE  c1 IS [NOT] NULL
+```
+
+### 使用 SQL 约束
+
+将c1和c2设置为主键
+
+```sql
+CREATE TABLE t(
+    c1 INT, c2 INT, c3 VARCHAR,
+    PRIMARY KEY (c1,c2)
+);
+```
+
+将c2列设置为外键
+
+```sql
+CREATE TABLE t1(
+    c1 INT PRIMARY KEY,  
+    c2 INT,
+    FOREIGN KEY (c2) REFERENCES t2(c2)
+);
+```
+
+使c1和c2中的值唯一
+
+```sql
+CREATE TABLE t(
+    c1 INT, c1 INT,
+    UNIQUE(c2,c3)
+);
+```
+
+确保c1>0和c1>=c2中的值
+
+```sql
+CREATE TABLE t(
+  c1 INT, c2 INT,
+  CHECK(c1> 0 AND c1 >= c2)
+);
+```
+c2列中的设置值不为NULL
+
+```sql
+CREATE TABLE t(
+     c1 INT PRIMARY KEY,
+     c2 VARCHAR NOT NULL
+);
+```
+
+### 修改数据
+
+在表格中插入一行
+
+```sql
+INSERT INTO t(column_list)
+VALUES(value_list);
+```
+
+在表格中插入多行
+
+```sql
+INSERT INTO t(column_list)
+VALUES (value_list), 
+       (value_list), …;
+```
+
+将行从t2插入t1
+
+```sql
+INSERT INTO t1(column_list)
+SELECT column_list
+FROM t2;
+```
+
+更新列c1中所有行的新值
+
+```sql
+UPDATE t
+SET c1 = new_value;
+```
+
+更新列c1、c2中与条件匹配的值
+
+```sql
+UPDATE t
+SET c1 = new_value, 
+        c2 = new_value
+WHERE condition;
+```
+
+删除表中的所有数据
+
+```sql
+DELETE FROM t;
+```
+
+删除表中的行子集
+
+```sql
+DELETE FROM t
+WHERE condition;
+```
+
+### 管理视图
+<!--rehype:wrap-class=row-span-2-->
+
+创建由c1和c2组成的新视图
+
+```sql
+CREATE VIEW v(c1,c2) 
+AS
+SELECT c1, c2
+FROM t;
+```
+
+使用选中选项创建新视图
+
+```sql
+CREATE VIEW v(c1,c2) 
+AS
+SELECT c1, c2
+FROM t;
+WITH [CASCADED | LOCAL] CHECK OPTION;
+```
+
+创建递归视图
+
+```sql
+CREATE RECURSIVE VIEW v 
+AS
+select-statement -- anchor part
+UNION [ALL]
+select-statement; -- recursive part
+```
+
+创建临时视图
+
+```sql
+CREATE TEMPORARY VIEW v 
+AS
+SELECT c1, c2
+FROM t;
+```
+
+删除视图
+
+```sql
+DROP VIEW view_name;
+```
+
+### 管理触发器
+
+创建或修改触发器
+
+```sql
+CREATE OR MODIFY TRIGGER trigger_name
+WHEN EVENT
+ON table_name TRIGGER_TYPE
+EXECUTE stored_procedure;
+```
+
+#### WHEN
+
+:-|:-
+:-|:-
+`BEFORE` | 在事件发生前调用
+`AFTER`  | 事件发生后调用
+
+#### EVENT
+
+:-|:-
+:-|:-
+`INSERT` | 为INSERT调用
+`UPDATE` | 调用UPDATE
+`DELETE` | 调用DELETE
+
+#### TRIGGER_TYPE
+
+:-|:-
+:-|:-
+`FOR EACH ROW`       | -
+`FOR EACH STATEMENT` | -
+
+### 管理索引
+
+在t表的c1和c2上创建索引
+
+```sql
+CREATE INDEX idx_name 
+ON t(c1,c2);
+```
+
+在t表的c3、c4上创建唯一索引
+
+```sql
+CREATE UNIQUE INDEX idx_name
+ON t(c3,c4)
+```
+
+删除索引
+
+```sql
+DROP INDEX idx_name;
+```
+
+MySQL 数据类型
+---------
+
+
+### Strings
+
+| -            | -                           |
+|--------------|-----------------------------|
+| `CHAR`       | String (0 - 255)            |
+| `VARCHAR`    | String (0 - 255)            |
+| `TINYTEXT`   | String (0 - 255)            |
+| `TEXT`       | String (0 - 65535)          |
+| `BLOB`       | String (0 - 65535)          |
+| `MEDIUMTEXT` | String (0 - 16777215)       |
+| `MEDIUMBLOB` | String (0 - 16777215)       |
+| `LONGTEXT`   | String (0 - 429496­7295)    |
+| `LONGBLOB`   | String (0 - 429496­7295)    |
+| `ENUM`       | One of preset options       |
+| `SET`        | Selection of preset options |
+
+
+### Date & time
+| Data Type   | Format              |
+|-------------|---------------------|
+| `DATE     ` | yyyy-MM-dd          |
+| `TIME     ` | hh:mm:ss            |
+| `DATETIME ` | yyyy-MM-dd hh:mm:ss |
+| `TIMESTAMP` | yyyy-MM-dd hh:mm:ss |
+| `YEAR     ` | yyyy                |
+
+
+### Numeric
+
+| -             | -                                                             |
+|---------------|---------------------------------------------------------------|
+| `TINYINT x`   | Integer (-128 to 127)                                         |
+| `SMALLINT x`  | Integer (-32768 to 32767)                                     |
+| `MEDIUMINT x` | Integer (-8388608 to 8388607)                                 |
+| `INT x`       | Integer (-2147­483648 to 214748­3647)                         |
+| `BIGINT x`    | Integer (-9223­372­036­854­775808 to 922337­203­685­477­5807) |
+| `FLOAT`       | Decimal (precise to 23 digits)                                |
+| `DOUBLE`      | Decimal (24 to 53 digits)                                     |
+| `DECIMAL`     | "­DOU­BLE­" stored as string                                  |
+
+另见
+---
+
+- [SQL 基础教程](http://www.w3school.com.cn/sql/index.asp) _(w3school.com.cn)_
+- [SQL 语句教程](http://www.1keydata.com/cn/sql/sql-count.php) _(1keydata.com)_
+- [21分钟 MySQL 基础入门](https://jaywcjlove.github.io/mysql-tutorial/21-minutes-MySQL-basic-entry.html) _(jaywcjlove.github.io)_