日期函数

--获取当前日期
mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2023-07-09     |
+----------------+
--获取当前时间
mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 09:42:21       |
+----------------+
--获取当前时间戳
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2023-07-09 09:42:27 |
+---------------------+
--获取当前日期时间
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2023-07-09 09:43:07 |
+---------------------+
--从日期时间中提取日期
mysql> select date(now());
+-------------+
| date(now()) |
+-------------+
| 2023-07-09  |
+-------------+
--在当前日期基础上增加10天
mysql> select date_add(now(),interval 10 day);
+---------------------------------+
| date_add(now(),interval 10 day) |
+---------------------------------+
| 2023-07-19 09:52:56             |
+---------------------------------+
--在当前日期基础上减少10天
mysql> select date_sub(now(),interval 10 day);
+---------------------------------+
| date_sub(now(),interval 10 day) |
+---------------------------------+
| 2023-06-29 09:53:04             |
+---------------------------------+
--获取时间差
mysql> select datediff(date(now()),'2002-05-01');
+------------------------------------+
| datediff(date(now()),'2002-05-01') |
+------------------------------------+
|                               7739 |
+------------------------------------+

综合案列

--创建msg表
mysql> create table msg (
id int primary key auto_increment,
content varchar(30) not null,
sendtime datetime
);
--插入两条数据
mysql> insert into msg(content,sendtime) values('天街小雨润如酥', now());
mysql> insert into msg(content,sendtime) values('草色遥看近却无', now());
--显示所有留言信息，发布日期只显示日期，不用显示时间
mysql> select * from msg;
+----+-----------------------+---------------------+
| id | content               | sendtime            |
+----+-----------------------+---------------------+
|  1 | 天街小雨润如酥        | 2023-07-09 10:20:25 |
|  2 | 草色遥看近却无        | 2023-07-09 10:20:36 |
+----+-----------------------+---------------------+

--查询在10分钟内发布的帖子
mysql> select * from msg where date_add(sendtime,interval 10  minute)>now();
+----+-----------------------+---------------------+
| id | content               | sendtime            |
+----+-----------------------+---------------------+
|  1 | 天街小雨润如酥        | 2023-07-09 10:20:25 |
|  2 | 草色遥看近却无        | 2023-07-09 10:20:36 |
+----+-----------------------+---------------------+
mysql> select * from msg where date_sub(now(),interval 10 minute)<sendtime;
+----+-----------------------+---------------------+
| id | content               | sendtime            |
+----+-----------------------+---------------------+
|  1 | 天街小雨润如酥        | 2023-07-09 10:20:25 |
|  2 | 草色遥看近却无        | 2023-07-09 10:20:36 |
+----+-----------------------+---------------------+

字符串函数

1.获取msg表的content列的字符集
2.拼接显示stu表中的信息
3.求学生表中学生姓名占用的字节数
length函数返回字符串长度，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是三个字节）
4.将stu表中所有数学成绩中有8的替换成’6’
5.截取stu表中name字段的第二个到第三个字符
6.以首字母小写的方式显示所有员工的姓名

mysql> select charset(content) from msg;
+------------------+
| charset(content) |
+------------------+
| utf8             |
| utf8             |
+------------------+

mysql> select concat('姓名：',name,'总分:',chinese+math+english,'语文',chinese,'数学',math,'英语',english) msg from stu;
+--------------------------------------------------------+
| msg                                                    |
+--------------------------------------------------------+
| 姓名：唐三藏总分:288语文134数学98英语56                |
| 姓名：猪悟能总分:364语文176数学98英语90                |
| 姓名：曹孟德总分:297语文140数学90英语67                |
| 姓名：刘玄德总分:270语文110数学115英语45               |
| 姓名：孙权总分:291语文140数学73英语78                  |
| 姓名：宋公明总分:275语文150数学95英语30                |
+--------------------------------------------------------+

mysql> select name,length(name) from stu;
+-----------+--------------+
| 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 |
+-----------+--------------+
mysql> select math,replace(math,'8','6') from stu;
+------+-----------------------+
| math | replace(math,'8','6') |
+------+-----------------------+
|   98 | 96                    |
|   98 | 96                    |
|   90 | 90                    |
|  115 | 115                   |
|   73 | 73                    |
|   95 | 95                    |
+------+-----------------------+
6 rows in set (0.00 sec)
--截取stu表中name字段的第二个到第三个字符
mysql> select name,substring(name,2,2) from stu;
+-----------+---------------------+
| 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                |
+-----------+---------------------+
--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                                                 |
+--------+--------------------------------------------------------+

数学函数

--取绝对值
mysql> select abs(10.2);
+-----------+
| abs(10.2) |
+-----------+
|      10.2 |
+-----------+
--取绝对值
mysql> select abs(-10.5);
+------------+
| abs(-10.5) |
+------------+
|       10.5 |
+------------+
--转为二进制
mysql> select bin(5);
+--------+
| bin(5) |
+--------+
| 101    |
+--------+
--对小数转二进制会自动取整
mysql> select bin(5.2);
+----------+
| bin(5.2) |
+----------+
| 101      |
+----------+

--转化为16进制
mysql> select hex(10);
+---------+
| hex(10) |
+---------+
| A       |
+---------+
--将十进制16转为2进制
mysql> select conv(16,10,2);
+---------------+
| conv(16,10,2) |
+---------------+
| 10000         |
+---------------+

--向上取整
mysql> select ceiling(5.2);
+--------------+
| ceiling(5.2) |
+--------------+
|            6 |
+--------------+
--向上取整
mysql> select ceiling(-5.2);
+---------------+
| ceiling(-5.2) |
+---------------+
|            -5 |
+---------------+
----向下取整
mysql> select floor(-5.2);
+-------------+
| floor(-5.2) |
+-------------+
|          -6 |
+-------------+
----向下取整
mysql> select floor(5.2);
+------------+
| floor(5.2) |
+------------+
|          5 |
+------------+

--保留2位小数位数（小数四舍五入)
mysql> select format(12.3569,2)
    -> ;
+-------------------+
| format(12.3569,2) |
+-------------------+
| 12.36             |
+-------------------+

--生成[0.0,10.0)之间的随机数
mysql> select rand()*10;
+-------------------+
| rand()*10         |
+-------------------+
| 3.120866942332002 |
+-------------------+
mysql> select format(rand()*10,2);
+---------------------+
| format(rand()*10,2) |
+---------------------+
| 8.73                |
+---------------------+
--取模
mysql> select mod(5,2);
+----------+
| mod(5,2) |
+----------+
|        1 |
+----------+

加密函数与其它

user() 查询当前用户
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
对于一个加密后的数据，想要查询它，也需要配套使用

insert into user (name,password) values ('wmh',md5('123'));
select * from user where password=md5('123');

mysql> select user();
+----------------+
| user()         |
+----------------+
| root@localhost |
+----------------+
mysql> select md5('root');
+----------------------------------+
| md5('root')                      |
+----------------------------------+
| 63a9f0ea7bb98050796b649e85481845 |
+----------------------------------+
--database()显示当前正在使用的数据库
mysql> select database();
+------------+
| database() |
+------------+
| scott      |
+------------+
--password()函数，MySQL数据库使用该函数对用户加密
mysql> select password('root');
+-------------------------------------------+
| password('root')                          |
+-------------------------------------------+
| *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B |
+-------------------------------------------+

--ifnull（val1， val2） 如果val1为null，返回val2，否则返回val1的值
mysql> select ifnull(null,10);
+-----------------+
| ifnull(null,10) |
+-----------------+
|              10 |
+-----------------+
mysql> select ifnull(20,null);
+-----------------+
| ifnull(20,null) |
+-----------------+
|              20 |
+-----------------+

---

解题思路: length(string) 统计出所有字符串的长度；减去没有逗号的长度，就是逗号的长度，这里需要用 '‘替换’,'了

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