【MySQL】内置函数

一、日期函数

--获取当前日期（年月日）
mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2023-07-28     |
+----------------+
1 row in set (0.02 sec)
--获取当前时间（时分秒）
mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 09:48:47       |
+----------------+
1 row in set (0.00 sec)
--获取时间戳（转化为 年月日 时分秒）
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2023-07-28 09:49:33 |
+---------------------+
1 row in set (0.00 sec)
-- 获取当前时间
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2023-07-28 09:50:05 |
+---------------------+
1 row in set (0.00 sec)
-- 从日期时间中提取日期
mysql> select date(now());
+-------------+
| date(now()) |
+-------------+
| 2023-07-28  |
+-------------+
1 row in set (0.00 sec)
-- 当前日期+10天
mysql> select date_add(now(),interval 10 day);
+---------------------------------+
| date_add(now(),interval 10 day) |
+---------------------------------+
| 2023-08-07 10:00:55             |
+---------------------------------+
1 row in set (0.00 sec)
-- 当前日期-10天
mysql> select date_sub(now(),interval 10 day);
+---------------------------------+
| date_sub(now(),interval 10 day) |
+---------------------------------+
| 2023-07-18 10:01:23             |
+---------------------------------+
1 row in set (0.01 sec)
-- 计算两个不同日期相差的天数
mysql> select datediff('2017-10-21','2017-06-01');
+-------------------------------------+
| datediff('2017-10-21','2017-06-01') |
+-------------------------------------+
|                                 142 |
+-------------------------------------+
1 row in set (0.00 sec)

综合案例1：

mysql> create table tmp( id int primary key auto_increment, birthday date not null);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into tmp(birthday) values('1990-10-10');
Query OK, 1 row affected (0.00 sec)

mysql> insert into tmp(birthday) values('1980-12-21');
Query OK, 1 row affected (0.01 sec)

mysql> select * from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 1990-10-10 |
|  2 | 1980-12-21 |
+----+------------+
2 rows in set (0.00 sec)

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

mysql> select * from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 1990-10-10 |
|  2 | 1980-12-21 |
|  3 | 2023-07-28 |
+----+------------+
3 rows in set (0.00 sec)

mysql> insert into tmp(birthday) values(current_time());
Query OK, 1 row affected, 1 warning (0.01 sec)

mysql> select * from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 1990-10-10 |
|  2 | 1980-12-21 |
|  3 | 2023-07-28 |
|  4 | 2023-07-28 |
+----+------------+
4 rows in set (0.00 sec)

综合案例2：

mysql> create table msg (
    -> id int primary key auto_increment,
    -> content varchar(30) not null,
    -> sendtime datetime
    -> );
Query OK, 0 rows affected (0.07 sec)

mysql> insert into msg(content,sendtime)values('好难啊',now());
Query OK, 1 row affected (0.01 sec)

mysql> insert into msg(content,sendtime)values('坚持住',now());
Query OK, 1 row affected (0.01 sec)

mysql> select * from msg;
+----+-----------+---------------------+
| id | content   | sendtime            |
+----+-----------+---------------------+
|  1 | 好难啊    | 2023-07-28 10:18:07 |
|  2 | 坚持住    | 2023-07-28 10:18:14 |
+----+-----------+---------------------+
2 rows in set (0.00 sec)

mysql> select content,date(sendtime) from msg;
+-----------+----------------+
| content   | date(sendtime) |
+-----------+----------------+
| 好难啊    | 2023-07-28     |
| 坚持住    | 2023-07-28     |
+-----------+----------------+
2 rows in set (0.00 sec)

select content,sendtime from msg where sendtime > date_sub(now(),interval 10 minute);
+-----------+---------------------+
| content   | sendtime            |
+-----------+---------------------+
| 好难啊    | 2023-07-28 10:18:07 |
| 坚持住    | 2023-07-28 10:18:14 |
+-----------+---------------------+
2 rows in set (0.00 sec)

mysql> select content,sendtime from msg where date_add(sendtime,interval 15 minute) > now();
+-----------+---------------------+
| content   | sendtime            |
+-----------+---------------------+
| 好难啊    | 2023-07-28 10:18:07 |
| 坚持住    | 2023-07-28 10:18:14 |
+-----------+---------------------+
2 rows in set (0.01 sec)

发帖子时间+10分钟 > 当前时间 说明发帖时间在10分钟内
当前时间-10分钟 < 发帖子时间 说明发帖时间在10分钟内

二、字符串函数

--查看字段编码格式
mysql> select charset(name) from exam_result;
+---------------+
| charset(name) |
+---------------+
| utf8          |
| utf8          |
| utf8          |
| utf8          |
| utf8          |
| utf8          |
+---------------+
6 rows in set (0.00 sec)
--字符串拼接
mysql> select concat('姓名:',name,'总分',chinese+math+english) from exam_result;
+------------------------------------------------------+
| concat('姓名:',name,'总分',chinese+math+english)     |
+------------------------------------------------------+
| 姓名:唐三藏总分288                                   |
| 姓名:猪悟能总分364                                   |
| 姓名:曹孟德总分297                                   |
| 姓名:刘玄德总分270                                   |
| 姓名:孙权总分291                                     |
| 姓名:宋公明总分275                                   |
+------------------------------------------------------+
6 rows in set (0.00 sec)
--计算字段所占用字节数
mysql> select name,length(name) from exam_result;
+-----------+--------------+
| name      | length(name) |
+-----------+--------------+
| 唐三藏    |            9 |
| 猪悟能    |            9 |
| 曹孟德    |            9 |
| 刘玄德    |            9 |
| 孙权      |            6 |
| 宋公明    |            9 |
+-----------+--------------+
6 rows in set (0.00 sec)
--是否在字符串中  在哪个位置
mysql> select instr('aaa1234','1234');
+-------------------------+
| instr('aaa1234','1234') |
+-------------------------+
|                       4 |
+-------------------------+
1 row in set (0.00 sec)
--小写转大写
mysql> select ucase('abc123ABC');
+--------------------+
| ucase('abc123ABC') |
+--------------------+
| ABC123ABC          |
+--------------------+
1 row in set (0.00 sec)
--大写转小写
mysql> select lcase('abc123ABC');
+--------------------+
| lcase('abc123ABC') |
+--------------------+
| abc123abc          |
+--------------------+
1 row in set (0.00 sec)
--从字符串左边拿n个字符
mysql> select left('aaabcd',3);
+------------------+
| left('aaabcd',3) |
+------------------+
| aaa              |
+------------------+
1 row in set (0.00 sec)
--从字符串右边拿n个字符
mysql> select right('aaabcd',3);
+-------------------+
| right('aaabcd',3) |
+-------------------+
| bcd               |
+-------------------+
1 row in set (0.00 sec)
--把字段中值的替换
mysql> select math,replace(math,'8','6') from exam_result;
+------+-----------------------+
| math | replace(math,'8','6') |
+------+-----------------------+
|   98 | 96                    |
|   98 | 96                    |
|   90 | 90                    |
|  115 | 115                   |
|   73 | 73                    |
|   95 | 95                    |
+------+-----------------------+
6 rows in set (0.00 sec)
--从某个字符开始截取多少个字符
mysql> select name,substring(name,2,2) from exam_result;
+-----------+---------------------+
| name      | substring(name,2,2) |
+-----------+---------------------+
| 唐三藏    | 三藏                |
| 猪悟能    | 悟能                |
| 曹孟德    | 孟德                |
| 刘玄德    | 玄德                |
| 孙权      | 权                  |
| 宋公明    | 公明                |
+-----------+---------------------+
6 rows in set (0.00 sec)
--以首字母小写的方式显示所有员工的姓名
mysql> select ename,concat(lcase(substring(ename,1,1)),substring(ename,2))  from emp;
+--------+--------------------------------------------------------+
| ename  | concat(lcase(substring(ename,1,1)),substring(ename,2)) |
+--------+--------------------------------------------------------+
| SMITH  | sMITH                                                  |
| ALLEN  | aLLEN                                                  |
| WARD   | wARD                                                   |
| JONES  | jONES                                                  |
| MARTIN | mARTIN                                                 |
| BLAKE  | bLAKE                                                  |
| CLARK  | cLARK                                                  |
| SCOTT  | sCOTT                                                  |
| KING   | kING                                                   |
| TURNER | tURNER                                                 |
| ADAMS  | aDAMS                                                  |
| JAMES  | jAMES                                                  |
| FORD   | fORD                                                   |
| MILLER | mILLER                                                 |
+--------+--------------------------------------------------------+
14 rows in set (0.00 sec)

--清理两边的空格
mysql> select trim('    你好    ')as res;
+--------+
| res    |
+--------+
| 你好   |
+--------+
1 row in set (0.00 sec)
--清理左边空格
mysql> select ltrim('    你好    ')as res;
+------------+
| res        |
+------------+
| 你好       |
+------------+
1 row in set (0.00 sec)
--清理右边空格
mysql> select rtrim('    你好    ')as res;
+------------+
| res        |
+------------+
|     你好   |
+------------+

三、数学函数

--绝对值
mysql> select abs(10),abs(-10);
+---------+----------+
| abs(10) | abs(-10) |
+---------+----------+
|      10 |       10 |
+---------+----------+
1 row in set (0.00 sec)
--转二进制
mysql> select bin(10),bin(20);
+---------+---------+
| bin(10) | bin(20) |
+---------+---------+
| 1010    | 10100   |
+---------+---------+
1 row in set (0.04 sec)
--转十六进制
mysql> select hex(10),hex(15);
+---------+---------+
| hex(10) | hex(15) |
+---------+---------+
| A       | F       |
+---------+---------+
1 row in set (0.00 sec)
--一个数字  n进制转m进制
mysql> select conv(10,10,2),conv(10,2,10);
+---------------+---------------+
| conv(10,10,2) | conv(10,2,10) |
+---------------+---------------+
| 1010          | 2             |
+---------------+---------------+
1 row in set (0.00 sec)
--向上取整
mysql> select ceiling(2.31),ceiling(3.14);
+---------------+---------------+
| ceiling(2.31) | ceiling(3.14) |
+---------------+---------------+
|             3 |             4 |
+---------------+---------------+
1 row in set (0.00 sec)
--向下取整
mysql> select floor(2.31),floor(3.14);
+-------------+-------------+
| floor(2.31) | floor(3.14) |
+-------------+-------------+
|           2 |           3 |
+-------------+-------------+
1 row in set (0.00 sec)
--格式化数字  去小数点后几位
mysql> select format(3.1415926,2);
+---------------------+
| format(3.1415926,2) |
+---------------------+
| 3.14                |
+---------------------+
1 row in set (0.00 sec)

--生成随机数
mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.6888377207227403 |
+--------------------+
1 row in set (0.00 sec)

mysql> select rand();
+---------------------+
| rand()              |
+---------------------+
| 0.17741031961274364 |
+---------------------+
1 row in set (0.00 sec)

mysql> select rand()*100;
+-------------------+
| rand()*100        |
+-------------------+
| 82.05384666291424 |
+-------------------+
1 row in set (0.00 sec)

mysql> select rand()*100;
+------------------+
| rand()*100       |
+------------------+
| 57.0461405041126 |
+------------------+
1 row in set (0.00 sec)

四、其他函数&&OJ

--user() 查询当前用户
mysql> select user();
+--------+
| user() |
+--------+
| zzg@   |
+--------+
1 row in set (0.00 sec)
--md5(str)对一个字符串进行md5摘要，摘要后得到一个32位字符串
mysql> select md5('admin');
+----------------------------------+
| md5('admin')                     |
+----------------------------------+
| 21232f297a57a5a743894a0e4a801fc3 |
+----------------------------------+
1 row in set (0.00 sec)
--password()函数，MySQL数据库使用该函数对用户加密
mysql> select password('admin');
+-------------------------------------------+
| password('admin')                         |
+-------------------------------------------+
| *4ACFE3202A5FF5CF467898FC58AAB1D615029441 |
+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)
--database()显示当前正在使用的数据库
mysql> select database();
+------------+
| database() |
+------------+
| scott      |
+------------+
1 row in set (0.00 sec)

mysql> select ifnull('abc','123');
+---------------------+
| ifnull('abc','123') |
+---------------------+
| abc                 |
+---------------------+
1 row in set (0.00 sec)
--ifnull（val1， val2） 如果val1为null，返回val2，否则返回val1的值
mysql> select ifnull(null,'123');
+--------------------+
| ifnull(null,'123') |
+--------------------+
| 123                |
+--------------------+
1 row in set (0.00 sec)

密码加密

--建表
mysql> create table user(
    -> id int primary key auto_increment,
    -> password char(32) not null);
Query OK, 0 rows affected (0.06 sec)
--插入数据
mysql> insert into user(password) values(md5('123'));
Query OK, 1 row affected (0.01 sec)
--查看数据
mysql> select * from user;
+----+----------------------------------+
| id | password                         |
+----+----------------------------------+
|  1 | 202cb962ac59075b964b07152d234b70 |
+----+----------------------------------+
1 row in set (0.00 sec)
--正常找密码找不到
mysql> select * from user where password='123';
Empty set (0.00 sec)
--md5()加密找才可以
mysql> select * from user where password=md5('123');
+----+----------------------------------+
| id | password                         |
+----+----------------------------------+
|  1 | 202cb962ac59075b964b07152d234b70 |
+----+----------------------------------+
1 row in set (0.00 sec)
--用password()加密更多,mysql自动
mysql> select password('1231');
+-------------------------------------------+
| password('1231')                          |
+-------------------------------------------+
| *78D02788CBC4FB05D363AC13FDABB60C106B1584 |
+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)

答案：

select id,length(string)-length(replace(string,',','')) cnt from strings