【MySQL】内置函数

👉函数👈

日期函数

• 获得年月日：

mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2023-05-16     |
+----------------+
1 row in set (0.00 sec)

• 获得时分秒：

mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 19:13:44       |
+----------------+
1 row in set (0.00 sec

• 获得时间戳：

mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2023-05-16 19:14:25 |
+---------------------+
1 row in set (0.01 sec)

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2023-05-16 19:38:20 |
+---------------------+
1 row in set (0.00 sec)

同时使用 current_date 和 current_time 就相当于 current_timestamp

mysql> select current_date() 日期, current_time() 时间;
+------------+----------+
| 日期       | 时间     |
+------------+----------+
| 2023-05-16 | 19:16:12 |
+------------+----------+
1 row in set (0.00 sec)

• 在日期的基础上加日期：

-- 加上十天
mysql> select date_add('2023-5-16', interval 10 day);
+----------------------------------------+
| date_add('2023-5-16', interval 10 day) |
+----------------------------------------+
| 2023-05-26                             |
+----------------------------------------+
1 row in set (0.00 sec)

-- 加上一年
mysql> select date_add('2023-5-16', interval 1 year);
+----------------------------------------+
| date_add('2023-5-16', interval 1 year) |
+----------------------------------------+
| 2024-05-16                             |
+----------------------------------------+
1 row in set (0.00 sec)

-- 加上一秒
mysql> select date_add('2023-5-16', interval 1 second);
+------------------------------------------+
| date_add('2023-5-16', interval 1 second) |
+------------------------------------------+
| 2023-05-16 00:00:01                      |
+------------------------------------------+
1 row in set (0.00 sec)

• 在日期的基础上减去时间：

-- 减去十天
mysql> select date_sub('2023-5-16', interval 10 day);
+----------------------------------------+
| date_sub('2023-5-16', interval 10 day) |
+----------------------------------------+
| 2023-05-06                             |
+----------------------------------------+
1 row in set (0.00 sec)

-- 减去一年
mysql> select date_sub('2023-5-16', interval 1 year);
+----------------------------------------+
| date_sub('2023-5-16', interval 1 year) |
+----------------------------------------+
| 2022-05-16                             |
+----------------------------------------+
1 row in set (0.00 sec)

-- 减去一秒
mysql> select date_sub('2023-5-16', interval 1 second);
+------------------------------------------+
| date_sub('2023-5-16', interval 1 second) |
+------------------------------------------+
| 2023-05-15 23:59:59                      |
+------------------------------------------+
1 row in set (0.00 sec)

• 计算两个日期之间相差多少天：

mysql> select datediff('2023-5-26', '2023-05-06');
+-------------------------------------+
| datediff('2023-5-26', '2023-05-06') |
+-------------------------------------+
|                                  20 |
+-------------------------------------+
1 row in set (0.00 sec)

mysql> select datediff(now(), '1949-10-01');
+-------------------------------+
| datediff(now(), '1949-10-01') |
+-------------------------------+
|                         26890 |
+-------------------------------+
1 row in set (0.00 sec)

• 获取时间戳中的日期

mysql> select date(now());
+-------------+
| date(now()) |
+-------------+
| 2023-05-16  |
+-------------+
1 row in set (0.00 sec)

mysql> select date(current_timestamp());
+---------------------------+
| date(current_timestamp()) |
+---------------------------+
| 2023-05-16                |
+---------------------------+
1 row in set (0.00 sec)

创建生日表

mysql> create table birthday(
    -> id int primary key auto_increment,
    -> d date not null,
    -> t timestamp);

mysql> desc birthday;
+-------+-----------+------+-----+-------------------+-----------------------------+
| Field | Type      | Null | Key | Default           | Extra                       |
+-------+-----------+------+-----+-------------------+-----------------------------+
| id    | int(11)   | NO   | PRI | NULL              | auto_increment              |
| d     | date      | NO   |     | NULL              |                             |
| t     | timestamp | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------+-----------+------+-----+-------------------+-----------------------------+
3 rows in set (0.00 sec)

mysql> insert into birthday (d) values('1949-10-01');
Query OK, 1 row affected (0.00 sec)

mysql> insert into birthday (d) values(current_date());
Query OK, 1 row affected (0.01 sec)

-- 插入数据时,timestamp类型会默认更新为当前的最新时间
mysql> select * from birthday;
+----+------------+---------------------+
| id | d          | t                   |
+----+------------+---------------------+
|  1 | 1949-10-01 | 2023-05-16 19:30:43 |
|  2 | 2023-05-16 | 2023-05-16 19:30:54 |
+----+------------+---------------------+
2 rows in set (0.00 sec)

-- 将t字段的类型更改为datetime类型
mysql> alter table birthday modify t datetime;
Query OK, 2 rows affected (0.05 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> desc birthday;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| id    | int(11)  | NO   | PRI | NULL    | auto_increment |
| d     | date     | NO   |     | NULL    |                |
| t     | datetime | YES  |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> select * from birthday;
+----+------------+---------------------+
| id | d          | t                   |
+----+------------+---------------------+
|  1 | 1949-10-01 | 2023-05-16 19:30:43 |
|  2 | 2023-05-16 | 2023-05-16 19:30:54 |
+----+------------+---------------------+
2 rows in set (0.00 sec)

mysql> insert into birthday (d, t) values(current_date(), current_timestamp());
Query OK, 1 row affected (0.01 sec)

mysql> select * from birthday;
+----+------------+---------------------+
| id | d          | t                   |
+----+------------+---------------------+
|  1 | 1949-10-01 | 2023-05-16 19:30:43 |
|  2 | 2023-05-16 | 2023-05-16 19:30:54 |
|  3 | 2023-05-16 | 2023-05-16 19:37:02 |
+----+------------+---------------------+
3 rows in set (0.00 sec)

创建留言表

mysql> create table msg(
    -> id int unsigned primary key auto_increment,
    -> content varchar(100) not null,
    -> sendtime datetime
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> desc msg;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| id       | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| content  | varchar(100)     | NO   |     | NULL    |                |
| sendtime | datetime         | YES  |     | NULL    |                |
+----------+------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

-- 插入数据
mysql> insert into msg (content, sendtime) values('莫道桑榆晚，为霞尚漫天', now());
Query OK, 1 row affected (0.01 sec)

mysql> insert into msg (content, sendtime) values('须知少年凌云志，曾许人间第一流', now());
Query OK, 1 row affected (0.00 sec)

-- 显示所有留言及留言的时间
mysql> select * from msg;
+----+-----------------------------------------------+---------------------+
| id | content                                       | sendtime            |
+----+-----------------------------------------------+---------------------+
|  1 | 莫道桑榆晚，为霞尚漫天                        | 2023-05-16 19:58:10 |
|  2 | 须知少年凌云志，曾许人间第一流                | 2023-05-16 19:59:30 |
+----+-----------------------------------------------+---------------------+
2 rows in set (0.00 sec)

-- 查询在两分钟内发布的帖子
mysql> insert into msg (content, sendtime) values('仰天长笑出门去，我辈岂是蓬蒿人', now());
Query OK, 1 row affected (0.01 sec)

mysql> select * from msg where date_add(sendtime, interval 2 minute) > now();
+----+-----------------------------------------------+---------------------+
| id | content                                       | sendtime            |
+----+-----------------------------------------------+---------------------+
|  3 | 仰天长笑出门去，我辈岂是蓬蒿人                | 2023-05-16 20:03:37 |
+----+-----------------------------------------------+---------------------+
1 row in set (0.00 sec)

-- 理解：
------------------------------|-----------|-------------|------------------
                            初始时间     now()       初始时间+2min

字符串函数

• 查看字符串的字符集

-- 此处字符串的字符集与配置文件的默认字符集有关
mysql> select charset('hello world!');
+-------------------------+
| charset('hello world!') |
+-------------------------+
| utf8                    |
+-------------------------+
1 row in set (0.00 sec)

mysql> show variables like 'character_set%';
+--------------------------+----------------------------+
| Variable_name            | Value                      |
+--------------------------+----------------------------+
| character_set_client     | utf8                       |
| character_set_connection | utf8                       |
| character_set_database   | utf8                       |
| character_set_filesystem | binary                     |
| character_set_results    | utf8                       |
| character_set_server     | utf8                       |
| character_set_system     | utf8                       |
| character_sets_dir       | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)

• 获取 emp 表的 ename 和 deptno 列的字符集

mysql> select charset(ename) from emp;
+----------------+
| charset(ename) |
+----------------+
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
+----------------+
14 rows in set (0.00 sec)

mysql> select charset(deptno) from emp;
+-----------------+
| charset(deptno) |
+-----------------+
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
| binary          |
+-----------------+
14 rows in set (0.00 sec)

• 数字能够直接使用二进制来表示，不需要再进行编码，所以数字的 charset 为 binary，而字符串可能需要进行编码，其 charset 与配置文件中的 charset 和建表时指定的 charset 有关。

• 当出现乱码问题时，可能是因为存储数据的编码和取出数据的编码是不一样的。出现乱码问题时，可能查看 charset 是否一致。

• 字符串拼接

mysql> select concat('hello ', 'world!');
+----------------------------+
| concat('hello ', 'world!') |
+----------------------------+
| hello world!               |
+----------------------------+
1 row in set (0.00 sec)

-- 尽管是数字也能够直接拼接
mysql> select concat('hello ', 123, ' ', '3.1415926');
+-----------------------------------------+
| concat('hello ', 123, ' ', '3.1415926') |
+-----------------------------------------+
| hello 123 3.1415926                     |
+-----------------------------------------+
1 row in set (0.00 sec)

• 显示 exam_result 表中的信息，显示格式：“XXX的语文XXX分，数学XXX分，英语XXX分

mysql> select concat(name, '的语文', chinese, '分,数学', math, '分,英语', english, '分') 全班成绩 from exam_result;
+-------------------------------------------------+
| 全班成绩                                        |
+-------------------------------------------------+
| 唐三藏的语文67分,数学98分,英语56分              |
| 孙悟空的语文87分,数学78分,英语77分              |
| 猪悟能的语文88分,数学98分,英语90分              |
| 曹孟德的语文82分,数学84分,英语67分              |
| 刘玄德的语文55分,数学85分,英语45分              |
| 孙权的语文70分,数学73分,英语78分                |
| 宋公明的语文75分,数学65分,英语30分              |
+-------------------------------------------------+
7 rows in set (0.00 sec)

• 求字符串的长度

mysql> select length('hello world!');
+------------------------+
| length('hello world!') |
+------------------------+
|                     12 |
+------------------------+
1 row in set (0.00 sec)

mysql> select length(3.1415926);
+-------------------+
| length(3.1415926) |
+-------------------+
|                 9 |
+-------------------+
1 row in set (0.00 sec)

-- 求出月薪超过两千的人的名字的长度
mysql> select ename, sal, length(ename) len from emp where sal>2000;
+-------+---------+------+
| ename | sal     | len  |
+-------+---------+------+
| JONES | 2975.00 |    5 |
| BLAKE | 2850.00 |    5 |
| CLARK | 2450.00 |    5 |
| SCOTT | 3000.00 |    5 |
| KING  | 5000.00 |    4 |
| FORD  | 3000.00 |    4 |
+-------+---------+------+
6 rows in set (0.00 sec)

length 函数返回的是字符串长度，以字节为单位。如果是多字节字符则计算多个字节数；如果是单字节字符则算作一个字节。比如：字母，数字算作一个字节，中文表示多个字节数（与字符集编码有关）。

-- utf-8编码的一个汉字占三个字节
mysql> select length('你好');
+------------------+
| length('你好')   |
+------------------+
|                6 |
+------------------+
1 row in set (0.00 sec)

• 获取字符串的子串

mysql> select substring('123456', 3);
+------------------------+
| substring('123456', 3) |
+------------------------+
| 3456                   |
+------------------------+
1 row in set (0.00 sec)

mysql> select substring('123456', 0);
+------------------------+
| substring('123456', 0) |
+------------------------+
|                        |
+------------------------+
1 row in set (0.00 sec)

mysql> select substring('123456', 3, 4);
+---------------------------+
| substring('123456', 3, 4) |
+---------------------------+
| 3456                      |
+---------------------------+
1 row in set (0.00 sec)

mysql> select substring('你好，世界', 1, 2);
+------------------------------------+
| substring('你好，世界', 1, 2)      |
+------------------------------------+
| 你好                               |
+------------------------------------+
1 row in set (0.00 sec)

mysql> select substring('你好，世界', 1, 4);
+------------------------------------+
| substring('你好，世界', 1, 4)      |
+------------------------------------+
| 你好，世                           |
+------------------------------------+
1 row in set (0.00 sec)

• 数据库的字符串下标是从 1 开始的，原因是使用数据库的人可能不是程序员，只是普通用户。
• substring 截取子串就是按照一个个字符来截取的，而不是按照字节来截取的。
• substring 的第一个参数是要截取的字符串，第二个参数是截取的起始位置，第三个参数是截取多少个字符。如果省略第三个参数，默认截取到字符串的最后一个字符。

• 字符串替换

-- 'xyz'是要被替换的字符串,'XYZ'是用来替换的字符串
-- 如果原字符串中没有要被替换的字符串,则无法完成替换
mysql> select replace('abcdxyz1234', 'xyz', 'XYZ');
+--------------------------------------+
| replace('abcdxyz1234', 'xyz', 'XYZ') |
+--------------------------------------+
| abcdXYZ1234                          |
+--------------------------------------+
1 row in set (0.00 sec)

-- replace是全部替换,而不是部分替换
mysql> select replace('abcdxyz1234xyz', 'xyz', 'XYZ');
+-----------------------------------------+
| replace('abcdxyz1234xyz', 'xyz', 'XYZ') |
+-----------------------------------------+
| abcdXYZ1234XYZ                          |
+-----------------------------------------+
1 row in set (0.00 sec)

• 大小写转换

mysql> select lcase('ABCD');
+---------------+
| lcase('ABCD') |
+---------------+
| abcd          |
+---------------+
1 row in set (0.00 sec)

mysql> select ucase('abcd');
+---------------+
| ucase('abcd') |
+---------------+
| ABCD          |
+---------------+
1 row in set (0.00 sec)

以首字母大写的方式显示所有员工的姓名

mysql> select concat(ucase(substring(ename, 1, 1)), lcase(substring(ename, 2))) name from emp;
+--------+
| name   |
+--------+
| Smith  |
| Allen  |
| Ward   |
| Jones  |
| Martin |
| Blake  |
| Clark  |
| Scott  |
| King   |
| Turner |
| Adams  |
| James  |
| Ford   |
| Miller |
+--------+
14 rows in set (0.00 sec)

• 查找子串

-- instr返回的是子串第一次出现的位置,没有出现则返回零
-- instr查找子串也是以字符为单位的,而不是以字节为单位
mysql> select instr('abcd1234', 'b');
+------------------------+
| instr('abcd1234', 'b') |
+------------------------+
|                      2 |
+------------------------+
1 row in set (0.00 sec)

mysql> select instr('abcd1234', 4);
+----------------------+
| instr('abcd1234', 4) |
+----------------------+
|                    8 |
+----------------------+
1 row in set (0.00 sec)

mysql> select instr('abcd1234', 'e');
+------------------------+
| instr('abcd1234', 'e') |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.00 sec)

找出名字中包含 TH 的员工

-- 使用通配符
mysql> select * from emp where ename like '%TH%';
+--------+-------+-------+------+---------------------+--------+------+--------+
| empno  | ename | job   | mgr  | hiredate            | sal    | comm | deptno |
+--------+-------+-------+------+---------------------+--------+------+--------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL |     20 |
+--------+-------+-------+------+---------------------+--------+------+--------+
1 row in set (0.00 sec)

-- 内置函数也可以出现在where子句中
mysql> select * from emp where instr(ename, 'TH');
+--------+-------+-------+------+---------------------+--------+------+--------+
| empno  | ename | job   | mgr  | hiredate            | sal    | comm | deptno |
+--------+-------+-------+------+---------------------+--------+------+--------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL |     20 |
+--------+-------+-------+------+---------------------+--------+------+--------+
1 row in set (0.00 sec)

• 字符串比较

-- 和C语言的strcmp函数一直,一直比较两个字符串中
-- 的字符直到比较出结果或者直到对方到字符串的末尾
-- 字符的大小是按照ASCII码进行比较的
mysql> select strcmp('abc', 'abcd');
+-----------------------+
| strcmp('abc', 'abcd') |
+-----------------------+
|                    -1 |
+-----------------------+
1 row in set (0.00 sec)

mysql> select strcmp('abcd', 'abcd');
+------------------------+
| strcmp('abcd', 'abcd') |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.00 sec)

mysql> select strcmp('abce', 'abcd');
+------------------------+
| strcmp('abce', 'abcd') |
+------------------------+
|                      1 |
+------------------------+
1 row in set (0.00 sec)

-- 找出岗位是CLERK的所有员工
mysql> select * from emp where strcmp(job, 'CLERK') = 0;
+--------+--------+-------+------+---------------------+---------+------+--------+
| empno  | ename  | job   | mgr  | hiredate            | sal     | comm | deptno |
+--------+--------+-------+------+---------------------+---------+------+--------+
| 007369 | SMITH  | CLERK | 7902 | 1980-12-17 00:00:00 |  800.00 | NULL |     20 |
| 007876 | ADAMS  | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
| 007900 | JAMES  | CLERK | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL |     10 |
+--------+--------+-------+------+---------------------+---------+------+--------+
4 rows in set (0.00 sec)

• 从左右截取字符串

mysql> select left('abcd', 1);
+-----------------+
| left('abcd', 1) |
+-----------------+
| a               |
+-----------------+
1 row in set (0.00 sec)

mysql> select left('abcd', 3);
+-----------------+
| left('abcd', 3) |
+-----------------+
| abc             |
+-----------------+
1 row in set (0.00 sec)

mysql> select right('abcd', 3);
+------------------+
| right('abcd', 3) |
+------------------+
| bcd              |
+------------------+
1 row in set (0.00 sec)

mysql> select right('abcd', 1);
+------------------+
| right('abcd', 1) |
+------------------+
| d                |
+------------------+
1 row in set (0.00 sec)

• 去重空格

-- trim不会去除字符串中间的空格
mysql> select trim('abcd 1234') ret;
+-----------+
| ret       |
+-----------+
| abcd 1234 |
+-----------+
1 row in set (0.00 sec)

-- trim会去除左右两边的空格
mysql> select trim('   abcd 1234  ') ret;
+-----------+
| ret       |
+-----------+
| abcd 1234 |
+-----------+
1 row in set (0.00 sec)

-- ltrim只会去除字符串左边的空格
mysql> select ltrim('   abcd 1234  ') ret;
+-------------+
| ret         |
+-------------+
| abcd 1234   |
+-------------+
1 row in set (0.00 sec)

-- rtrim只会去除字符串右边的空格
mysql> select rtrim('   abcd 1234  ') ret;
+--------------+
| ret          |
+--------------+
|    abcd 1234 |
+--------------+
1 row in set (0.00 sec)

数学函数

• 绝对值

mysql> select abs(-10);
+----------+
| abs(-10) |
+----------+
|       10 |
+----------+
1 row in set (0.00 sec)

mysql> select abs(-3.14);
+------------+
| abs(-3.14) |
+------------+
|       3.14 |
+------------+
1 row in set (0.00 sec)

• 十进制转二进制

mysql> select bin(10);
+---------+
| bin(10) |
+---------+
| 1010    |
+---------+
1 row in set (0.00 sec)

mysql> select bin(-1);
+------------------------------------------------------------------+
| bin(-1)                                                          |
+------------------------------------------------------------------+
| 1111111111111111111111111111111111111111111111111111111111111111 |
+------------------------------------------------------------------+
1 row in set (0.00 sec)

• 十进制转十六进制

mysql> select hex(10);
+---------+
| hex(10) |
+---------+
| A       |
+---------+
1 row in set (0.00 sec)

mysql> select hex(32);
+---------+
| hex(32) |
+---------+
| 20      |
+---------+
1 row in set (0.00 sec)

• 进制转换

-- 将十进制的10转成二进制
mysql> select conv(10, 10, 2);
+-----------------+
| conv(10, 10, 2) |
+-----------------+
| 1010            |
+-----------------+
1 row in set (0.00 sec)

-- 将二进制的10转成十进制
mysql> select conv(10, 2, 10);
+-----------------+
| conv(10, 2, 10) |
+-----------------+
| 2               |
+-----------------+
1 row in set (0.00 sec)

• 向上取整

mysql> select ceiling(3.14);
+---------------+
| ceiling(3.14) |
+---------------+
|             4 |
+---------------+
1 row in set (0.00 sec)

mysql> select ceiling(-3.14);
+----------------+
| ceiling(-3.14) |
+----------------+
|             -3 |
+----------------+
1 row in set (0.00 sec)

• 向下取整

mysql> select floor(3.14);
+-------------+
| floor(3.14) |
+-------------+
|           3 |
+-------------+
1 row in set (0.00 sec)

mysql> select floor(-3.14);
+--------------+
| floor(-3.14) |
+--------------+
|           -4 |
+--------------+
1 row in set (0.00 sec)

• 格式化

-- 保留2位小数位数（小数四舍五入)
mysql> select format(12.3456, 2);
+--------------------+
| format(12.3456, 2) |
+--------------------+
| 12.35              |
+--------------------+
1 row in set (0.00 sec)

mysql> select format(100, 2);
+----------------+
| format(100, 2) |
+----------------+
| 100.00         |
+----------------+
1 row in set (0.00 sec)

• 随机数

-- rand函数随机产生[0.0, 1.0)之间的数
mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.6638928788377837 |
+--------------------+
1 row in set (0.00 sec)

mysql> select rand();
+---------------------+
| rand()              |
+---------------------+
| 0.21991669872767916 |
+---------------------+
1 row in set (0.00 sec)

-- 对rand函数进行加减乘除就可以拼出任意区间的随机数
mysql> select rand()-rand();
+---------------------+
| rand()-rand()       |
+---------------------+
| -0.7718694859853661 |
+---------------------+
1 row in set (0.00 sec)

-- 产生[0, 100)之间的随机数
mysql> select 100*rand();
+------------------+
| 100*rand()       |
+------------------+
| 45.6843777053844 |
+------------------+
1 row in set (0.00 sec)

rand 函数可以配合 format 函数随机出整数。

• 取模

mysql> select mod(1, 3);
+-----------+
| mod(1, 3) |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql> select mod(-1, 3);
+------------+
| mod(-1, 3) |
+------------+
|         -1 |
+------------+
1 row in set (0.00 sec)

mysql> select mod(3.14, 2);
+--------------+
| mod(3.14, 2) |
+--------------+
|         1.14 |
+--------------+
1 row in set (0.00 sec)

注：有关负数取模的原则，可以自行查询，本人就不赘述了。

其它函数

• 查询当前用户

mysql> select user();
+----------------+
| user()         |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)

查看 user 表

mysql> use mysql;
mysql> select * from user;
-- 查看user表结构
mysql> desc user;

user 函数的实现原理

-- user函数可以通过select加concat函数来实现
mysql> select concat(User, '@', Host) `user()` from user where User='root';
+----------------+
| user()         |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)

• md5 函数

mysql> select md5('a');
+----------------------------------+
| md5('a')                         |
+----------------------------------+
| 0cc175b9c0f1b6a831c399e269772661 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> select md5('ab');
+----------------------------------+
| md5('ab')                        |
+----------------------------------+
| 187ef4436122d1cc2f40dc2b92f0eba0 |
+----------------------------------+
1 row in set (0.00 sec)

• md5 算法对一个字符串进行 md5 摘要，摘要后等到一个 32 位的字符串，不管原字符串是多长。
• 尽管两个字符串只相差一个字符，md5 算法形成的两个摘要相差也很大。
• md5 算法可以对秘密进行摘要，从而起到用户的密码，因为很难通过 md5 算法形成的摘要破解出原密码。

-- authentication_string 是密码通过md5算法形成的摘要
mysql> select user, host, authentication_string from user;
mysql> select user, host, authentication_string from user;
+---------------+-----------+-------------------------------------------+
| user          | host      | authentication_string                     |
+---------------+-----------+-------------------------------------------+
| root          | localhost | *4A4A4E1077BBA88E4537392A2D56040FF27F1FC1 |
| mysql.session | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.sys     | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
+---------------+-----------+-------------------------------------------+
3 rows in set (0.00 sec)

模拟用户注册的场景文章来源地址https://www.toymoban.com/news/detail-613054.html

-- 在自己的数据库中进行模拟
mysql> select database();
+------------+
| database() |
+------------+
| my_db      |
+------------+
1 row in set (0.00 sec)

-- 创建用户表
mysql> create table if not exists user(
    -> id bigint unsigned primary key auto_increment,
    -> name varchar(32) not null,
    -> password char(64) not null,
    -> reg_time datetime not null
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> insert into user (name, password, reg_time) values('张三', md5('123456'), now());
Query OK, 1 row affected (0.00 sec)

mysql> select * from user;
+----+--------+----------------------------------+---------------------+
| id | name   | password                         | reg_time            |
+----+--------+----------------------------------+---------------------+
|  1 | 张三   | e10adc3949ba59abbe56e057f20f883e | 2023-05-16 23:17:41 |
+----+--------+----------------------------------+---------------------+
1 row in set (0.00 sec)

mysql> select * from user where md5('123456') = password;
+----+--------+----------------------------------+---------------------+
| id | name   | password                         | reg_time            |
+----+--------+----------------------------------+---------------------+
|  1 | 张三   | e10adc3949ba59abbe56e057f20f883e | 2023-05-16 23:17:41 |
+----+--------+----------------------------------+---------------------+
1 row in set (0.00 sec)

• 用户表中储存的密码都不是真正用户的密码，都是摘要来的，因此后端进行用户认证比较的都是摘要。
• 涉及到 md5 算法和密码等私密的信息,无法通过上翻来找到历史输过的 SQL 语句。

• password 函数

-- MySQL数据库使用password函数对用户加密
mysql> select password('123456');
+-------------------------------------------+
| password('123456')                        |
+-------------------------------------------+
| *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> select length(password('123456'));
+----------------------------+
| length(password('123456')) |
+----------------------------+
|                         41 |
+----------------------------+
1 row in set, 1 warning (0.00 sec)

• ifnull 函数

-- ifnull（val1, val2） 如果val1为null,返回val2,否则返回val1的值
-- ifnull就相当于是三目运算符

mysql> select ifnull('a', 'b');
+------------------+
| ifnull('a', 'b') |
+------------------+
| a                |
+------------------+
1 row in set (0.00 sec)

mysql> select ifnull(null, 'b');
+-------------------+
| ifnull(null, 'b') |
+-------------------+
| b                 |
+-------------------+
1 row in set (0.00 sec)

mysql> select ifnull('a', null);
+-------------------+
| ifnull('a', null) |
+-------------------+
| a                 |
+-------------------+
1 row in set (0.00 sec)

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