日期函数

规定：日期：年月日时间：时分秒

函数名称	作用描述
current_date()	当前日期
current_time()	当前时间
current_timestamp()	当前时间戳
date(datetime)	返回datetime参数的日期部分
date_add(date,interval d_value_type)	在date中添加时间或日期。interval后面可以是year、day、minute、second
date_sub(date,interval d_value_type)	在date中减去时间或日期。interval后面可以是year、day、minute、second
datediff(date1,date2)	两个日期的时间差，单位是天
now()	当前时间日期

函数使用演示

获得年月日：

mysql> select current_date;
+--------------+
| current_date |
+--------------+
| 2023-08-16   |
+--------------+
1 row in set (0.00 sec)

获得时分秒：

mysql> select current_time;
+--------------+
| current_time |
+--------------+
| 23:37:33     |
+--------------+
1 row in set (0.00 sec)

获得时间戳：

mysql> select current_timestamp;
+---------------------+
| current_timestamp   |
+---------------------+
| 2023-08-16 23:38:11 |
+---------------------+
1 row in set (0.00 sec)

在日期的基础上加日期：

mysql> select date_add(now(),interval 10 day);  ---加10天
+---------------------------------+
| date_add(now(),interval 10 day) |
+---------------------------------+
| 2023-08-26 23:39:04             |
+---------------------------------+
1 row in set (0.00 sec)

mysql> select now();-----当前的日期天数
+---------------------+
| now()               |
+---------------------+
| 2023-08-16 23:39:11 |
+---------------------+
1 row in set (0.00 sec)

在日期的基础上减去时间：

mysql> select now();-----原本的日期
+---------------------+
| now()               |
+---------------------+
| 2023-08-16 23:40:08 |
+---------------------+
1 row in set (0.00 sec)

mysql> select date_sub(now(),interval 5 day);----5天前
+--------------------------------+
| date_sub(now(),interval 5 day) |
+--------------------------------+
| 2023-08-11 23:40:26            |
+--------------------------------+
1 row in set (0.00 sec)

计算两个日期之间相差多少天：

mysql> select datediff('2019-12-31','2023-8-16');
+------------------------------------+
| datediff('2019-12-31','2023-8-16') |
+------------------------------------+
|                              -1324 |
+------------------------------------+
1 row in set (0.00 sec)

mysql> select datediff('2023-8-16','2019-12-31');
+------------------------------------+
| datediff('2023-8-16','2019-12-31') |
+------------------------------------+
|                               1324 |
+------------------------------------+
1 row in set (0.00 sec)

案例

1.创建一张表，记录生日，添加当前日期：

mysql> create table tmp(
    -> id int primary key auto_increment,
    -> birthday date
    -> );
Query OK, 0 rows affected (0.03 sec)

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

mysql> select * from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 2023-08-16 |
+----+------------+
1 row in set (0.00 sec)

2.创建一个留言表，插入相关数据。①显示所有留言信息，发布日期只显示日期，不用显示时间②查询在2分钟内发布的帖子。

建表

mysql> create table msg(
    -> id int primary key auto_increment,
    -> content varchar(30) not null,
    -> sendtime datetime);
Query OK, 0 rows affected (0.03 sec)

插入数据

mysql> insert into msg(content,sendtime) values('中午吃什么？',now());
Query OK, 1 row affected (0.00 sec)

mysql> insert into msg(content,sendtime) values('我想吃螺蛳粉，可以吗',now());
Query OK, 1 row affected (0.00 sec)

mysql> select * from msg;
+----+--------------------------------+---------------------+
| id | content                        | sendtime            |
+----+--------------------------------+---------------------+
|  1 | 中午吃什么？                   | 2023-08-16 23:51:57 |
|  2 | 我想吃螺蛳粉，可以吗           | 2023-08-16 23:52:09 |
+----+--------------------------------+---------------------+
2 rows in set (0.00 sec)

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 content,date(sendtime) from msg;
+--------------------------------+----------------+
| content                        | date(sendtime) |
+--------------------------------+----------------+
| 中午吃什么？                   | 2023-08-16     |
| 我想吃螺蛳粉，可以吗           | 2023-08-16     |
+--------------------------------+----------------+
2 rows in set (0.00 sec)

请查询在2分钟内发布的帖子

mysql> insert into msg(content,sendtime) values('项目做了吗？',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 | 项目做了吗？       | 2023-08-16 23:56:26 |
+----+--------------------+---------------------+
1 row in set (0.00 sec)

字符串函数

案例：

获取stu表的 name的字符集----使用charset字符串函数

stu表：

mysql> desc stu;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | NO   | PRI | NULL    |       |
| name     | varchar(30) | NO   |     | NULL    |       |
| class_id | int(11)     | YES  | MUL | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> select charset(name) from stu;  ----获取字符串的字符集
+---------------+
| charset(name) |
+---------------+
| utf8          |
| utf8          |
+---------------+
2 rows in set (0.00 sec)

要求显示exam_result表中的信息，显示格式：“XXX的语文是XXX分，数学XXX分，英语XXX分”----使用concat字符串函数

mysql> desc exam_result
    -> ;
+---------+------------------+------+-----+---------+----------------+
| Field   | Type             | Null | Key | Default | Extra          |
+---------+------------------+------+-----+---------+----------------+
| id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| name    | varchar(20)      | NO   |     | NULL    |                |
| chinese | float            | YES  |     | 0       |                |
| math    | float            | YES  |     | 0       |                |
| english | float            | YES  |     | 0       |                |
| qq      | char(10)         | YES  |     | NULL    |                |
+---------+------------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)

mysql> select concat(name,'的语文是',chinese,'分, 数学是',math,'分') as '分数' from exam_result;
+----------------------------------------------+
| 分数                                         |
+----------------------------------------------+
| 唐三藏的语文是134分, 数学是98分              |
| 猪悟能的语文是176分, 数学是98分              |
| 曹孟德的语文是140分, 数学是90分              |
| 刘玄德的语文是110分, 数学是115分             |
| 孙权的语文是140分, 数学是73分                |
| 宋公明的语文是150分, 数学是95分              |
+----------------------------------------------+
6 rows in set (0.00 sec)

求exam_result中学生姓名占用的字节数---使用length字符串函数

注意：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）。

mysql> select length(name),name from exam_result;
+--------------+-----------+
| length(name) | name      |
+--------------+-----------+
|            9 | 唐三藏    |
|            9 | 猪悟能    |
|            9 | 曹孟德    |
|            9 | 刘玄德    |
|            6 | 孙权      |
|            9 | 宋公明    |
+--------------+-----------+
6 rows in set (0.00 sec)

以首字母小写的方式显示所有同学的姓名

select concat(lcase(substring(name,1,1)),substring(name,2)) from exam_result;

数学函数

案例：

绝对值

mysql> select abs(-100.2);
+-------------+
| abs(-100.2) |
+-------------+
|       100.2 |
+-------------+
1 row in set (0.00 sec)

向上取整

mysql> select ceiling(23.04);
+----------------+
| ceiling(23.04) |
+----------------+
|             24 |
+----------------+
1 row in set (0.00 sec)

向下取整

mysql> select floor(23.7);
+-------------+
| floor(23.7) |
+-------------+
|          23 |
+-------------+
1 row in set (0.00 sec)

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)

产生随机数

mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.3399681042320729 |
+--------------------+
1 row in set (0.00 sec)

mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.5697201356009768 |
+--------------------+
1 row in set (0.00 sec)

mysql> select rand();
+------------------+
| rand()           |
+------------------+
| 0.82869639604231 |
+------------------+
1 row in set (0.00 sec)

其它函数

user() 查询当前用户

select user();

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)

database()显示当前正在使用的数据库

mysql> select database();
+------------+
| database() |
+------------+
| my_test    |
+------------+
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('123456');
+-------------------------------------------+
| password('123456')                        |
+-------------------------------------------+
| *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+-------------------------------------------+
1 row in set, 1 warning (0.02 sec)

ifnull（val1， val2）如果val1为null，返回val2，否则返回val1的值

mysql> select ifnull('abc','123');
+---------------------+
| ifnull('abc','123') |
+---------------------+
| abc                 |
+---------------------+
1 row in set (0.00 sec)

mysql> select ifnull(null,'123');
+--------------------+
| ifnull(null,'123') |
+--------------------+
| 123                |
+--------------------+
1 row in set (0.00 sec)