SQL语句

SQL语句有多种类型：

数据查询语言（DQL）
数据查询语言（Data Query Language, DQL）是SQL语言中，负责进行数据查询而不会对数据本身进行修改的语句，这是最基本的SQL语句。SELECT是DQL（也是所有SQL）用得最多的动词，其他DQL常用的保留字有FROM，WHERE，GROUP BY，HAVING和ORDER BY。这些DQL保留字常与其他类型的SQL语句一起使用。
数据定义语言（DDL）
数据定义语言 (Data Definition Language, DDL) 是SQL语言集中，负责数据结构定义与数据库对象定义的语言，由CREATE、ALTER与DROP三个语法所组成。
数据操纵语言（DML）
数据操纵语言（Data Manipulation Language, DML）是SQL语言中，负责对数据库对象运行数据访问工作的指令集，以INSERT、UPDATE、DELETE三种指令为核心，分别代表插入、更新与删除。
数据控制语言（DCL）
数据控制语言 (Data Control Language) 在SQL语言中，是一种可对数据访问权进行控制的指令，它可以控制特定用户账户对数据表、查看表、预存程序、用户自定义函数等数据库对象的控制权。由GRANT和REVOKE两个指令组成。DCL以控制用户的访问权限为主，GRANT为授权语句，对应的REVOKE是撤销授权语句。
指针控制语言（CCL）
它的语句，像DECLARE CURSOR，FETCH INTO和UPDATE WHERE CURRENT用于对一个或多个表单独行的操作。
事务处理语言（TPL）
它的语句能确保被DML语句影响的表的所有行及时得以更新。TPL语句包括BEGIN TRANSACTION，COMMIT和ROLLBACK。

1、DDL操作

1.1 数据库DDL操作

//创建数据库
//语法：CREATE DATABASE [IF NOT EXISTS] 'DB_NAME';

//创建数据库hzz
mysql> CREATE DATABASE IF NOT EXISTS hzz;
Query OK, 1 row affected (0.00 sec)

//查看当前实例有哪些数据库
mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| hzz	             |
+--------------------+
5 rows in set (0.00 sec)

//删除数据库
//语法：DROP DATABASE [IF EXISTS] 'DB_NAME';

//删除数据库hzz
mysql> DROP DATABASE IF EXISTS hzz;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
4 rows in set (0.00 sec)

//创建数据库时,设置数据库的字符集：
//CHARACTER SET:指定数据库采用的字符集,utf8不能写成utf-8,建议使用utf8mb4字符集
//COLLATE:指定数据库字符集的排序规则,utf8mb4的默认排序规则为utf8mb4_general_ci（通过show character set查看）
mysql> CREATE DATABASE dbtest CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

//数据库客户端字符编码需要和服务端字符集保持一致
//SET NAMES:指定客户端字符集
mysql> SET NAMES utf8mb4;

//字符集配置都可写入MySQL配置文件中启动MySQL服务时自动加载

1.2 表DDL操作

//创建表
//语法：CREATE TABLE table_name (col1 datatype 修饰符,col2 datatype 修饰符) ENGINE='存储引擎类型';

//在数据库hzz里创建表hzztable
mysql> CREATE DATABASE hzz;      	//创建数据库hzz
Query OK, 1 row affected (0.00 sec)

mysql> use hzz;      			//进入hzz数据库
Database changed

//创建hzztable表
mysql> CREATE TABLE hzztable (id int(10) NOT NULL,name VARCHAR(100) NOT NULL,age tinyint);
Query OK, 0 rows affected (0.09 sec)

//查看当前数据库有哪些表
mysql> SHOW TABLES;
+----------------------+
| Tables_in_hzz |
+----------------------+
| hzztable             |
+----------------------+
1 row in set (0.00 sec)

//删除表
//语法：DROP TABLE [ IF EXISTS ] 'table_name';

//删除表hzztable
mysql> DROP TABLE hzztable;
Query OK, 0 rows affected (0.01 sec)

mysql> SHOW TABLES;
Empty set (0.00 sec)

//创建表时，设置字段、表的字符编码
mysql> CREATE TABLE tbtest(id int(10),user_name varchar(60) CHARACTER SET GBK COLLATE gbk_chinese_ci,email varchar(60))CHARACTER SET utf8mb4 COLLATE utf8_general_ci;

1.3 用户DDL操作

MySQL用户帐号由两部分组成，如'USERNAME'@'HOST'，表示此USERNAME只能从此HOST上远程登录；HOST用于限制此用户可通过哪些主机远程连接mysql程序。

HOST的值可为：

IP地址，如：172.16.12.129
通配符
- %：匹配任意长度的任意字符，常用于设置允许从任何主机登录
- _：匹配任意单个字符

//数据库用户创建
//语法：CREATE USER 'username'@'host' [IDENTIFIED BY 'password'];

//创建数据库用户hzz
mysql> CREATE USER 'hzz'@'127.0.0.1' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected (0.00 sec)

//刷新授权表
mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.006 sec)

//使用新创建的用户和密码登录
[root@localhost ~]# mysql -uhzz -phzz123! -h127.0.0.1
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.7.23 MySQL Community Server (GPL)

Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> 

//删除数据库用户
//语法：DROP USER 'username'@'host'; 

//删除数据库用户hzz
mysql> DROP USER 'hzz'@'127.0.0.1';
Query OK, 0 rows affected (0.00 sec)

//刷新授权表
mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.006 sec)

1.4 查看命令SHOW

mysql> SHOW CHARACTER SET;      	//查看支持的所有字符集
+----------+---------------------------------+---------------------+--------+
| Charset  | Description                     | Default collation   | Maxlen |
+----------+---------------------------------+---------------------+--------+
| big5     | Big5 Traditional Chinese        | big5_chinese_ci     |      2 |
| dec8     | DEC West European               | dec8_swedish_ci     |      1 |
| cp850    | DOS West European               | cp850_general_ci    |      1 |
| hp8      | HP West European                | hp8_english_ci      |      1 |
| koi8r    | KOI8-R Relcom Russian           | koi8r_general_ci    |      1 |
| latin1   | cp1252 West European            | latin1_swedish_ci   |      1 | 
......
......

mysql> SHOW VARIABLES LIKE '%char%';	//查看客户端的字符编码
.....
.....

mysql> SELECT CHARSET(email) FROM tbtest;	//查看某表中某字段使用的字符编码
.....
.....

mysql> SHOW ENGINES;        		//查看当前数据库支持的所有存储引擎
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment
                 | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| InnoDB             | DEFAULT | Supports transactions, row-level locking, and
foreign keys     | YES          | YES  | YES        |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables
                 | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
| MyISAM             | YES     | MyISAM storage engine                                          | NO           | NO   | NO         |
| CSV                | YES     | CSV storage engine                                            | NO           | NO   | NO         |
| ARCHIVE            | YES     | Archive storage engine                                        | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                            | NO           | NO   | NO         |
| FEDERATED          | NO      | Federated MySQL storage engine                                | NULL         | NULL | NULL       |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)

mysql> SHOW DATABASES;          	//查看数据库信息
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| hzz                |
+--------------------+
5 rows in set (0.00 sec)

mysql> SHOW TABLES FROM hzz;         	//不进入某数据库而列出其包含的所有表
+----------------------+
| Tables_in_hzz        |
+----------------------+
| hzztable             |
+----------------------+
1 row in set (0.00 sec)

//查看表结构
//语法：DESC [db_name.]table_name;

mysql> DESC hzz.hzztable;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | NO   |     | NULL    |       |
| name  | varchar(100) | NO   |     | NULL    |       |
| age   | tinyint(4)   | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.03 sec)

//查看某表的创建命令，可以看到创建表时设置的参数
//语法：SHOW CREATE TABLE table_name;

mysql> SHOW CREATE TABLE hzz.hzztable;
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                              |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+
| hzztable | CREATE TABLE `hzz` (
  `id` int(11) NOT NULL,
  `name` varchar(100) NOT NULL,
  `age` tinyint(4) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

//查看某表的状态
//语法：SHOW TABLE STATUS LIKE 'table_name'\G

mysql> use hzz;      //进入数据库hzz
Database changed
mysql> SHOW TABLE STATUS LIKE 'hzztable'\G      //查看hzztable表的状态
*************************** 1. row ***************************
           Name: hzztable
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 0
 Avg_row_length: 0
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2018-08-13 00:53:21
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)

1.5 ALTER修改操作

//修改数据库属性
//语法：ALTER DATABASE 'DB_NAME' CHARACTER SET charset_name | COLLATE collation_name

//修改数据库字符集为utf8
mysql> ALTER DATABASE hzz CHARACTER SET utf8;
Query OK, 1 row affected (0.000 sec)

//修改表
//语法：ALTER TABLE <table_name> [option]

//给hzztable表添加新的一列，先查看表结构
mysql> DESC hzztable;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(7)      | NO   |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
| age   | tinyint(4)  | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.001 sec)

//给hzztable表添加一列'phone'
mysql> ALTER TABLE hzztable ADD phone int(11);
Query OK, 0 rows affected (0.001 sec)
Records: 0  Duplicates: 0  Warnings: 0

//查看表结构
mysql> DESC hzztable;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(7)      | NO   |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
| age   | tinyint(4)  | YES  |     | NULL    |       |
| phone | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.000 sec)

//修改hzztable表中'phone'列为'sex'
mysql> ALTER TABLE hzztable CHANGE phone sex varchar(4);
Query OK, 0 rows affected (0.005 sec)              
Records: 0  Duplicates: 0  Warnings: 0

mysql> DESC hzztable;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(7)      | NO   |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
| age   | tinyint(4)  | YES  |     | NULL    |       |
| sex   | varchar(4)  | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.001 sec)

//修改表名为zzhtable
mysql> ALTER TABLE hzztable RENAME TO zzhtable;
Query OK, 0 rows affected (0.002 sec)

mysql> SHOW TABLES;
+---------------+
| Tables_in_hzz |
+---------------+
| zzhtable      |
+---------------+
1 row in set (0.000 sec)

//修改数据库字符编码
mysql> ALTER DATABASE dbtest CHARACTER SET utf8 COLLATE utf8_general_ci;

//修改表字符编码
mysql> ALTER TABLE tbtest CHARACTER SET utf8 COLLATE utf8_general_ci;

1.6 获取帮助

//获取命令使用帮助
//语法：HELP commond;

mysql> HELP CREATE TABLE;       //获取创建表的帮助
Name: 'CREATE TABLE'
Description:
Syntax:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    (create_definition,...)
    [table_options]
    [partition_options]

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    [(create_definition,...)]
    [table_options]
    [partition_options]
    [IGNORE | REPLACE]
    [AS] query_expression
 ......
 ......

2、DML操作

DML操作包括增(INSERT)、删(DELETE)、改(UPDATE)、查(SELECT)，均属针对表的操作。

2.1 INSERT语句

//DML操作之增操作insert
//语法：INSERT [INTO] table_name [(column_name,...)] {VALUES | VALUE} (value1,...),(...),...

mysql> use hzz;
Database changed

//一次插入一条记录
mysql> INSERT INTO hzztable (id,name,age) VALUE (1,'tom',20);       
Query OK, 1 row affected (0.01 sec)

//一次插入多条记录
mysql> INSERT INTO hzztable (id,name,age) VALUES (2,'jerry',23),(3,'hzz',25),(4,'sean',28),(5,'zhangshan',26),(6,'zhangshan',20),(7,'lisi',NULL);
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0

2.2 SELECT语句(DQL)

字段column匹配方式:

表示符	含义
*	所有字段
as	字段别名，如col1 AS alias1 当表名很长时用别名代替

条件判断语句WHERE:

操作类型	常用操作符
操作符	>，<，>=，<=，=，!= BETWEEN column# AND column# LIKE：模糊匹配 RLIKE：基于正则表达式进行模式匹配 IS NOT NULL：非空 IS NULL：空
条件逻辑操作	AND OR NOT

//DML操作之查操作select
//语法：SELECT column1,column2,... FROM table_name [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];

mysql> use hzz;
Database changed

mysql> select * from hzztable;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | tom       |   20 |
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
|  6 | zhangshan |   20 |
|  7 | lisi      | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)

mysql> SELECT name FROM hzztable;
+-----------+
| name      |
+-----------+
| tom       |
| jerry     |
| hzz       |
| sean      |
| zhangshan |
| zhangshan |
| lisi      |
+-----------+
7 rows in set (0.01 sec)

mysql> SELECT * FROM hzztable WHERE age >= 25;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
+----+-----------+------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM hzztable WHERE age >= 25 AND name = 'zhangshan';
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  5 | zhangshan |   26 |
+----+-----------+------+
1 row in set (0.00 sec)

mysql> SELECT * FROM hzztable WHERE age BETWEEN 23 and 28;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
+----+-----------+------+
4 rows in set (0.00 sec)

mysql> select * from hzztable where age is not null;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | tom       |   20 |
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
|  6 | zhangshan |   20 |
+----+-----------+------+
6 rows in set (0.00 sec)

mysql> select * from hzztable where age is null;
+----+------+------+
| id | name | age  |
+----+------+------+
|  7 | lisi | NULL |
+----+------+------+
1 row in set (0.00 sec)

mysql> select * from hzztable where name like '%z%';
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  3 | hzz       |   25 |
|  5 | zhangshan |   26 |
|  6 | zhangshan |   20 |
+----+-----------+------+
3 rows in set (0.00 sec)

GROUP BY 分组聚合函数

group by 分组功能原理：
1.按照group by指定的列进⾏排序；
2.然后根据group by指定的列去重复，也就是将相同的值分成一组；
3.最后通过聚合函数将其他列的结果进⾏聚合。

聚合函数

count() 统计数量
sum() 求和
avg() 平均数
max() 最⼤值
min() 最⼩值
group_concat() 列转⾏

//首先以name字段进行分组，然后对该字段进行排序、去重，最后通过count()聚合函数将name字段对应的age字段的数量统计出来
mysql> select name,count(age) from hzztable group by name;
+-----------+-------------+
| name      | count(age)  |
+-----------+-------------+
| hzz       |      1      |
| jerry     |      1      |
| sean      |      1      |
| tom       |      1      |
| zhangshan |      2      |
+----+-----------+--------+
5 rows in set (0.00 sec)

GROUP BY 分组聚合函数流程图

GROUP BY 分组聚合函数流程图

HAVING 聚合判断

HAVING：聚合判断，主要应⽤在GROUP BY之后再次进行判断。

select name,count(age) from hzztable group by name having count(age)>=2;
+-----------+-------------+
| name      | count(age)  |
+-----------+-------------+
| zhangshan |      2      |
+----+-----------+--------+

ORDER BY 聚合排序

ORDER BY：聚合排序，默认为升序(ASC)，也可降序(DESC)。

ORDER BY语句	含义
ORDER BY ‘column_name'	根据column_name进行升序排序
ORDER BY 'column_name' DESC	根据column_name进行降序排序

mysql> SELECT * FROM hzztable ORDER BY age;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  7 | lisi      | NULL |
|  1 | tom       |   20 |
|  6 | zhangshan |   20 |
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  5 | zhangshan |   26 |
|  4 | sean      |   28 |
+----+-----------+------+
7 rows in set (0.00 sec)

mysql> SELECT * FROM hzztable ORDER BY age DESC;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
|  3 | hzz       |   25 |
|  2 | jerry     |   23 |
|  1 | tom       |   20 |
|  6 | zhangshan |   20 |
|  7 | lisi      | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)

LIMIT 分⻚查询

LIMIT：分⻚查询，主要对数据进行分页，查询想要的内容。

LIMIT语句	含义
ORDER BY ’column_name' LIMIT 2	根据column_name进行升序排序并只取前2个结果
ORDER BY ‘column_name' LIMIT 1,2	根据column_name进行升序排序并且略过第1个结果取后面的2个结果

mysql> SELECT * FROM hzztable ORDER BY age limit 2;
+----+------+------+
| id | name | age  |
+----+------+------+
|  7 | lisi | NULL |
|  1 | tom  |   20 |
+----+------+------+
2 rows in set (0.00 sec)

mysql> SELECT * FROM hzztable ORDER BY age limit 1,2;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | tom       |   20 |
|  6 | zhangshan |   20 |
+----+-----------+------+
2 rows in set (0.00 sec)

2.3 update语句

//DML操作之改操作update
//语法：UPDATE table_name SET column1 = new_value1[,column2 = new_value2,...] [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];

mysql> select * from hzztable;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | tom       |   20 |
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
|  6 | zhangshan |   20 |
|  7 | lisi      | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)

mysql> update hzztable set age = 30 where name = 'lisi';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from hzztable where name = 'lisi';
+----+------+------+
| id | name | age  |
+----+------+------+
|  7 | lisi |   30 |
+----+------+------+
1 row in set (0.00 sec)

2.4 delete语句

//DML操作之删操作delete
//语法：DELETE FROM table_name [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];

mysql> select * from hzztable;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | tom       |   20 |
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
|  6 | zhangshan |   20 |
|  7 | lisi      |   30 |
+----+-----------+------+
7 rows in set (0.00 sec)

mysql> delete from hzztable where id = 7;       //删除某条记录
Query OK, 1 row affected (0.00 sec)

mysql> select * from hzztable;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | tom       |   20 |
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
|  6 | zhangshan |   20 |
+----+-----------+------+
6 rows in set (0.00 sec)

mysql> delete from hzztable;        //删除整张表的内容
Query OK, 6 rows affected (0.00 sec)

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

mysql> desc hzztable;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | NO   |     | NULL    |       |
| name  | varchar(100) | NO   |     | NULL    |       |
| age   | tinyint(4)   | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

2.5 truncate语句

truncate与delete的区别：

语句类型	特点
delete	DELETE删除表内容时仅删除内容，但会保留表结构 DELETE语句每次删除一行，并在事务日志中为所删除的每行记录一项可以通过回滚事务日志恢复数据非常占用空间
truncate	删除表中所有数据，且无法恢复表结构、约束和索引等保持不变，新添加的行计数值重置为初始值执行速度比DELETE快，且使用的系统和事务日志资源少通过释放存储表数据所用的数据页来删除数据，并且只在事务日志中记录页的释放对于有外键约束引用的表，不能使用TRUNCATE TABLE删除数据不能用于加入了索引视图的表

语句类型

特点

delete

DELETE删除表内容时仅删除内容，但会保留表结构
DELETE语句每次删除一行，并在事务日志中为所删除的每行记录一项
可以通过回滚事务日志恢复数据
非常占用空间

truncate

删除表中所有数据，且无法恢复
表结构、约束和索引等保持不变，新添加的行计数值重置为初始值
执行速度比DELETE快，且使用的系统和事务日志资源少
通过释放存储表数据所用的数据页来删除数据，并且只在事务日志中记录页的释放
对于有外键约束引用的表，不能使用TRUNCATE TABLE删除数据
不能用于加入了索引视图的表

//语法：TRUNCATE table_name;

mysql> select * from hzztable;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | tom       |   20 |
|  2 | jerry     |   23 |
|  3 | hzz       |   25 |
|  4 | sean      |   28 |
|  5 | zhangshan |   26 |
|  6 | zhangshan |   20 |
|  7 | lisi      | NULL |
+----+-----------+------+
7 rows in set (0.00 sec)

mysql> truncate hzztable;
Query OK, 0 rows affected (0.01 sec)

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

mysql> desc hzztable;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | NO   |     | NULL    |       |
| name  | varchar(100) | NO   |     | NULL    |       |
| age   | tinyint(4)   | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

3、DCL操作

3.1 创建授权grant

权限类型(priv_type)

权限类型	含义
ALL	所有权限
SELECT	读取内容的权限
INSERT	插入内容的权限
UPDATE	更新内容的权限
DELETE	删除内容的权限
....	....

指定要操作的对象db_name.table_name

表示方式	含义
.	所有库的所有表
db_name	指定库的所有表
db_name.table_name	指定库的指定表

WITH GRANT OPTION：被授权的用户可将自己的权限副本转赠给其他用户，说白点就是将自己的权限完全复制给另一个用户。不建议使用。

//语法：GRANT priv_type,... ON [object_type] db_name.table_name TO ‘username'@'host' [IDENTIFIED BY 'password'] [WITH GRANT OPTION];

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

//授权hzz用户在数据库本机上登录访问所有数据库
mysql> GRANT ALL ON *.* TO 'hzz'@'localhost' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)

//也可表示为：
mysql> GRANT ALL ON *.* TO 'hzz'@'127.0.0.1' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)

//授权hzz用户在172.16.12.129上远程登录访问hzz数据库
mysql> GRANT ALL ON hzz.* TO 'hzz'@'172.16.12.129' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)

//授权hzz用户在所有位置上远程登录访问hzz数据库
mysql> GRANT ALL ON hzz.* TO 'hzz'@'%' IDENTIFIED BY 'hzz123!';
Query OK, 0 rows affected, 1 warning (0.00 sec)

//刷新授权表
mysql> FLUSH PRIVILEGES;

3.2 查看授权

//查看当前登录用户的授权信息
mysql> SHOW GRANTS;
+---------------------------------------------------------------------+
| Grants for root@localhost                                           |
+---------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION |
| GRANT PROXY ON ''@'' TO 'root'@'localhost' WITH GRANT OPTION        |
+---------------------------------------------------------------------+
2 rows in set (0.00 sec)

//查看指定用户hzz的授权信息
mysql> SHOW GRANTS FOR 'hzz'@'localhost';
+-------------------------------------------------------+
| Grants for hzz@localhost                              |
+-------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'hzz'@'localhost'      |
+-------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SHOW GRANTS FOR 'hzz'@'127.0.0.1';
+-------------------------------------------------------+
| Grants for hzz@127.0.0.1                              |
+-------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'hzz'@'127.0.0.1'      |
+-------------------------------------------------------+
1 row in set (0.00 sec)

3.3 取消授权REVOKE

//语法：REVOKE priv_type,... ON db_name.table_name FROM 'username'@'host';

mysql> REVOKE ALL ON *.* FROM 'hzz'@'172.16.12.129';
Query OK, 0 rows affected (0.00 sec)

mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.00 sec)

注意：mysql服务进程启动时会读取mysql库中的所有授权表至内存中：

GRANT或REVOKE等执行权限操作会保存于表中，mysql的服务进程会自动重读授权表，并更新至内存中
对于不能够或不能及时重读授权表的命令，可手动让mysql的服务进程重读授权表

mysql> FLUSH PRIVILEGES;

4、找回root密码

1.停⽌MySQL服务
[root@localhost ~]# systemctl stop mysqld

2.编辑MySQL服务配置文件，跳过授权登陆验证
[root@localhost ~]# vim /etc/my.cnf
[mysqld]
skip-grant-tables	#添加此行，表示跳过授权表

3.开启MySQL服务，免密登陆MySQL
[root@localhost ~]# systemctl start mysqld
[root@localhost ~]# mysql	#直接使用mysql命令即可登录

4.登录成功后修改MySQL密码
mysql> UPDATE mysql.user SET authentication_string=PASSWORD("123456") WHERE user='root' and host='localhost';

5.关闭MySQL服务，删除配置文件中添加的跳过授权表配置
[root@localhost ~]# systemctl stop mysqld
[root@localhost ~]# vim /etc/my.cnf
[mysqld]
skip-grant-tables	#删除该行

6.再启动MySQL服务，使⽤修改后的密码登陆
[root@localhost ~]# systemctl start mysqld
[root@localhost ~]# mysql -uroot -p123456

7.注意
跳过授权表之后，所有的本地和远程⽤户都不需要⽤户名密码认证即可连接，⾮常危险。

MySQL SQL语句