【MySQL】内置函数
作者：@阿亮joy.
专栏：《零基础入门MySQL》
座右铭：每个优秀的人都有一段沉默的时光，那段时光是付出了很多努力却得不到结果的日子，我们把它叫做扎根
目录
-
- 函数
-
- 日期函数
- 字符串函数
- 数学函数
- 其它函数
- 总结
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
-- 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)
总结
本篇博客主要讲解了日期函数、字符串函数、数学函数、md5 函数、password 函数以及 ifnull 函数等等。以上就是本篇博客的全部内容了，如果大家觉得有收获的话，可以点个三连支持一下！谢谢大家啦！❣️