[MySQL基础3] 数据控制语言DCL和MySQL中的常用函数

DCL，即Data Control Language，用来管理数据库的用户和其访问权限

1. 用户管理

用户查询：

用户都在MySQL自带的user数据库中

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mysql> use mysql;

mysql> select User from user;
+------------------+
| User             |
+------------------+
| mysql.infoschema |
| mysql.session    |
| mysql.sys        |
| root             |
+------------------+
4 rows in set (0.00 sec)

创建用户：

一个用户由用户名和主机名（localhost）定义

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create user 用户名@主机名 identified by 密码;

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mysql> create user jack@localhost identified by '123';
Query OK, 0 rows affected (0.01 sec)

mysql> select User from user;
+------------------+
| User             |
+------------------+
| jack             |
| mysql.infoschema |
| mysql.session    |
| mysql.sys        |
| root             |
+------------------+
5 rows in set (0.00 sec)

mysql> -- 在任意主机访问数据库
mysql> create user amy@'%' identified by '123';
Query OK, 0 rows affected (0.01 sec)

mysql> select User from user;
+------------------+
| User             |
+------------------+
| amy              |
| jack             |
| mysql.infoschema |
| mysql.session    |
| mysql.sys        |
| root             |
+------------------+
6 rows in set (0.00 sec)

修改用户密码

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alter user 用户名@主机名 identified with mysql_native_password by 新密码

其中mysql_native_password意为MySQL的本地密码

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mysql> alter user amy identified with mysql_native_password by 'abc';
Query OK, 0 rows affected (0.01 sec)

删除用户

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drop user 用户名@主机名

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mysql> drop user jack@localhost;
Query OK, 0 rows affected (0.00 sec)

mysql> drop user amy@'%';
Query OK, 0 rows affected (0.00 sec)

mysql> select User from user;
+------------------+
| User             |
+------------------+
| mysql.infoschema |
| mysql.session    |
| mysql.sys        |
| root             |
+------------------+
4 rows in set (0.00 sec)

2. 权限控制

常用的权限有以下几种：

名称	说明
all, all privileges	所有权限
select	查询数据
insert	插入数据
update	修改数据
delete	删除数据
alter	修改表
drop	删除数据库/表/视图
create	创建数据库/表

基本的常用命令：

查询权限：
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```
show grants for 用户名@主机名
```
授予权限：
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```
grant 权限列表 on 数据库名.表名 to 用户名@主机名
```
注意, 授予和撤销都是要指定哪个数据库的哪个表数据库名.表名

撤销权限:

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revoke 权限列表 on 数据库名.表名 from 用户名@主机名

例子：

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mysql> show grants for jack@localhost;
+------------------------------------------+
| Grants for jack@localhost                |
+------------------------------------------+
| GRANT USAGE ON *.* TO `jack`@`localhost` |
+------------------------------------------+

-- 上面的意思是没有任何权限 仅能登录

-- 授予jack对info.user表格的所有权限
mysql> grant all on info.user to jack@localhost;
Query OK, 0 rows affected (0.01 sec)

-- 查看权限
mysql> show grants for jack@localhost;
+-------------------------------------------------------------+
| Grants for jack@localhost                                   |
+-------------------------------------------------------------+
| GRANT USAGE ON *.* TO `jack`@`localhost`                    |
| GRANT ALL PRIVILEGES ON `info`.`user` TO `jack`@`localhost` |
+-------------------------------------------------------------+

-- 登录jack用户后 可以查看到info数据库
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| info               |
| information_schema |
| performance_schema |
+--------------------+

-- 撤销jack用户权限(在root下)
mysql> revoke all on info.user from jack@localhost;
Query OK, 0 rows affected (0.01 sec)

-- 这时jack能看到的数据库
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| performance_schema |
+--------------------+
2 rows in set (0.00 sec)

-- 授予jack特定的权限
mysql> grant select, insert, update on info.user to jack@localhost;
Query OK, 0 rows affected (0.01 sec)

-- 从jack登录，可以插入，查询，更改表的数据
mysql> use info;
Database changed

mysql> insert into user values (1, 'aaa', 'F'), (2, 'bbb', 'M');
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from user where gender='F';
+------+------+--------+
| id   | name | gender |
+------+------+--------+
|    1 | aaa  | F      |
+------+------+--------+

mysql> update user set name='qwe' where id=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from user;
+------+------+--------+
| id   | name | gender |
+------+------+--------+
|    1 | qwe  | F      |
|    2 | bbb  | M      |
+------+------+--------+
2 rows in set (0.00 sec)

-- 但是没有删除表的权限
mysql> drop table user;
ERROR 1142 (42000): DROP command denied to user 'jack'@'localhost' for table 'user'

介绍一下MySQL中的函数，主要分为四大类：字符串函数、数值函数、日期函数和流程函数

1. 字符串函数

MySQL中常用的内置字符串函数如下：

函数	功能
`concat(str1, ..., strn)`	拼接字符串, 相当于Java StringBuilder的`append()`
`lower(str)`	转换为小写, 相当于python的`lower()`
`upper(str)`	转换为大写, 相当于python的`upper()`
`lpad(str, n, pad)`	用pad左填充至长度n, 相当于格式化字符串`'%{pad}{n}s'.format(str)`
`rpad(str, n, pad)`	用pad右填充至长度n
`trim(str)`	去掉字符串头尾空格, 相当于python的`strip()`
`substring(str, st, len)`	去从下标st开始长度为len的子串, 相当于C++的`substr()`. 注意索引从1开始!

例子:

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-- 用select做到打印内容
mysql> select concat('ab', 'cd');
+--------------------+
| concat('ab', 'cd') |
+--------------------+
| abcd               |
+--------------------+

mysql> select lower('asda'), upper('fdf');
+---------------+--------------+
| lower('asda') | upper('fdf') |
+---------------+--------------+
| asda          | FDF          |
+---------------+--------------+

mysql> select lpad('123', 10, '-'), rpad('123', 10, '*');
+----------------------+----------------------+
| lpad('123', 10, '-') | rpad('123', 10, '*') |
+----------------------+----------------------+
| -------123           | 123*******           |
+----------------------+----------------------+

mysql> select trim('  asdsd3  ');
+--------------------+
| trim('  asdsd3  ') |
+--------------------+
| asdsd3             |
+--------------------+

mysql> select substring('qwerty', 2, 3);
+---------------------------+
| substring('qwerty', 2, 3) |
+---------------------------+
| wer                       |
+---------------------------+

一个小案例：将user表中name的前面补0，至10位数。

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mysql> select * from user;
+------+------+--------+
| id   | name | gender |
+------+------+--------+
|    1 | qwe  | F      |
|    2 | bbb  | M      |
+------+------+--------+


mysql> update user set name=lpad(name, 5, '0');
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from user;
+------+-------+--------+
| id   | name  | gender |
+------+-------+--------+
|    1 | 00qwe | F      |
|    2 | 00bbb | M      |
+------+-------+--------+

2. 数值函数

MySQL中常用的内置数值函数如下，跟绝大多数语言的用法都是一样的：

函数	功能
`ceil(x)`	向上取整，`Java: Math.ceil(), Python: math.ceil()`
`floor(str)`	向下取整，`Java: Math.floor(), Python: math.floor(),`
`mod(x, y)`	计算x % y
`rand()`	0~1内随机数
`round(x, y)`	四舍五入x, 保留y位小数

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mysql> select ceil(1.3), floor(3.3), round(1/3, 3);
+-----------+------------+---------------+
| ceil(1.3) | floor(3.3) | round(1/3, 3) |
+-----------+------------+---------------+
|         2 |          3 |         0.333 |
+-----------+------------+---------------+
1 row in set (0.01 sec)

mysql> select mod(5, 2), rand();
+-----------+--------------------+
| mod(5, 2) | rand()             |
+-----------+--------------------+
|         1 | 0.5711282347991394 |
+-----------+--------------------+
1 row in set (0.01 sec)

3. 日期函数

MySQL中常用的日期函数如下：

函数	功能
`curdate()`	当前日期
`curtime()`	当前时间
`now()`	当前日期和时间
`year(date), month(date), day(date)`	获取指定date的年份,月份,日
`date_add(date, interval expr type)`	返回一个日期/时间值加上一个时间间隔expr后的时间值
`datediff(date1, date2)`	返回date1和date2之间的天数

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mysql> select curdate(), curtime(), now();
+------------+-----------+---------------------+
| curdate()  | curtime() | now()               |
+------------+-----------+---------------------+
| 2025-07-19 | 15:31:28  | 2025-07-19 15:31:28 |
+------------+-----------+---------------------+

mysql> select year('2024-3-3'), month('2025-04-21'), day(now());
+------------------+---------------------+------------+
| year('2024-3-3') | month('2025-04-21') | day(now()) |
+------------------+---------------------+------------+
|             2024 |                   4 |         19 |
+------------------+---------------------+------------+

mysql> select date_add(now(), interval 2 day), date_add(curdate(), interval 3 month), date_add(now(), interval 1 year);
+---------------------------------+---------------------------------------+----------------------------------+
| date_add(now(), interval 2 day) | date_add(curdate(), interval 3 month) | date_add(now(), interval 1 year) |
+---------------------------------+---------------------------------------+----------------------------------+
| 2025-07-21 15:34:59             | 2025-10-19                            | 2026-07-19 15:34:59              |
+---------------------------------+---------------------------------------+----------------------------------+

mysql> select date_add(now(), interval 2 second);
+------------------------------------+
| date_add(now(), interval 2 second) |
+------------------------------------+
| 2025-07-19 15:39:36                |
+------------------------------------+
1 row in set (0.00 sec)

mysql> select date_add(now(), interval 2 week);
+----------------------------------+
| date_add(now(), interval 2 week) |
+----------------------------------+
| 2025-08-02 15:39:37              |
+----------------------------------+
1 row in set (0.00 sec)

mysql> select datediff('2025-3-2 13:32:19', now());
+--------------------------------------+
| datediff('2025-3-2 13:32:19', now()) |
+--------------------------------------+
|                                 -139 |
+--------------------------------------+

案例：为user表格中增加日期列，并按照日期从早到晚排序查询

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mysql> alter table user add column entrydate date;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

-- 设定数据的时间，并且再增加一列
mysql> update user set entrydate='2025-01-01' where id=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> update user set entrydate=curdate() where id=2;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> insert into user values (3, 'asdasd', 'F', '2020-4-2');
Query OK, 1 row affected (0.01 sec)

-- 查看一下
mysql> select * from user;
+------+--------+--------+------------+
| id   | name   | gender | entrydate  |
+------+--------+--------+------------+
|    1 | qwe    | F      | 2025-01-01 |
|    2 | bbb    | M      | 2025-07-19 |
|    3 | asdasd | F      | 2020-04-02 |
+------+--------+--------+------------+
3 rows in set (0.00 sec)

-- 排序查询 日期从早到晚也就是距离今天从大到小
mysql> select name, datediff(curdate(), entrydate) as d from user order by d desc;
+--------+------+
| name   | d    |
+--------+------+
| asdasd | 1934 |
| qwe    |  199 |
| bbb    |    0 |
+--------+------+
3 rows in set (0.00 sec)

4. 流程控制函数

MySQL中常用的流程控制函数如下：

函数	功能
`if(value, t, f)`	如果value是true，返回t，否则f，相当于Java/C++三元运算符`value ? t : f;`
`ifnull(v1, v2)`	如果v1不是null返回v1，否则返回v2，相当于Java/C++`v1 == null ? v2 : v1;`
`case when [val1] then [ret1]...else [defalut] end`	如果val1为true则返回ret1，以此类推，最终返回default
`case [expr] when [val1] then [ret1]...else [defalut] end`	如果expr的值为val1则返回ret1，以此类推，最终返回default. 相当于Java/C++的`switch...case...default`

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mysql> select if(datediff(now(), '2024-3-4') > 0, 'yes', 'no');
+--------------------------------------------------+
| if(datediff(now(), '2024-3-4') > 0, 'yes', 'no') |
+--------------------------------------------------+
| yes                                              |
+--------------------------------------------------+

mysql> select ifnull(null, '123');
+---------------------+
| ifnull(null, '123') |
+---------------------+
| 123                 |
+---------------------+

-- 实现需求：查询表user，如果性别是F，就显示'女'，性别是M，就显示'男'
mysql> select name,
    -> (case gender when 'F' then '女' when 'M' then '男' else '不男不女' end) as gen
    -> from user;
+--------+-----+
| name   | gen |
+--------+-----+
| qwe    | 女  |
| bbb    | 男  |
| asdasd | 女  |
+--------+-----+
3 rows in set (0.01 sec)