【MySQL--07】内置函数

【MySQL–07】内置函数

1.函数

1.1日期函数

函数名称	描述
current_data()	当前日期
current_time()	当前时间
current_timestamp()	当前时间戳
date(datetime)	返回datetume参数的日期部分
date_add(date,interval d_value_type)	在date中添加日期时间 interval后的数值单位可以是： year_minute second day
date_sub(data,interval d_value_type)	在date中减去日期或时间 interval后的数值单位可以是： year minute second day
datadiff(date1,date2)	两个日期的差,单位是天
now()	当前日期时间

• 获得年月日：

mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2023-05-04     |
+----------------+
1 row in set (0.14 sec)

• 获得时分秒：

mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 17:30:28       |
+----------------+
1 row in set (0.01 sec)

• 获得时间戳：

mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2023-05-04 17:31:29 |
+---------------------+
1 row in set (0.03 sec)

• 在日期的基础上加日期：

mysql> select date_add('2023-5-1',interval 10 day);
+--------------------------------------+
| date_add('2023-5-1',interval 10 day) |
+--------------------------------------+
| 2023-05-11                           |
+--------------------------------------+
1 row in set (0.01 sec)

--可以重命名
mysql> select date_add('2023-5-1',interval 10 day) as result;
+------------+
| result     |
+------------+
| 2023-05-11 |
+------------+
1 row in set (0.01 sec)

--as可以省略
mysql> select date_add('2023-5-1',interval 10 day) result;
+------------+
| result     |
+------------+
| 2023-05-11 |
+------------+
1 row in set (0.00 sec)

• 在日期的基础上减日期：

mysql> select date_sub('2023-5-1',interval 10 day);
+--------------------------------------+
| date_sub('2023-5-1',interval 10 day) |
+--------------------------------------+
| 2023-04-21                           |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> select date_sub('2023-5-1',interval 10 day) as result;
+------------+
| result     |
+------------+
| 2023-04-21 |
+------------+
1 row in set (0.00 sec)

mysql> select date_sub('2023-5-1',interval 10 day) result;
+------------+
| result     |
+------------+
| 2023-04-21 |
+------------+
1 row in set (0.00 sec)

• 计算两个日期之间相差多少天

mysql> select datediff('2023-5-1','2023-6-1');
+---------------------------------+
| datediff('2023-5-1','2023-6-1') |
+---------------------------------+
|                             -31 |
+---------------------------------+
1 row in set (0.03 sec)

mysql> select datediff('2023-6-1','2023-5-1');
+---------------------------------+
| datediff('2023-6-1','2023-5-1') |
+---------------------------------+
|                              31 |
+---------------------------------+
1 row in set (0.00 sec)

我们可能够发现是前面的日期减去后面的日期的时间差

1.1.1案例1

• 创建一张表，记录生日

mysql> create table tmp(
    -> id int primary key auto_increment,
    -> birthday date);
Query OK, 0 rows affected (0.29 sec)

• 添加当前日期

mysql> insert into tmp(birthday) values (current_date());
Query OK, 1 row affected (0.04 sec)

mysql> select * from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 2023-05-04 |
+----+------------+
1 row in set (0.00 sec)

1.1.2案例2

• 创建一个留言表

mysql> create table msg(
    -> id int primary key auto_increment,
    -> content varchar(20) not null,
    -> sendtime datetime);
Query OK, 0 rows affected (0.23 sec)

• 插入数据

mysql> insert into msg(content,sendtime) values ('hello1',now());
Query OK, 1 row affected (0.04 sec)

mysql> insert into msg(content,sendtime) values ('hello2',now());
Query OK, 1 row affected (0.05 sec)

mysql> select * from msg;
+----+---------+---------------------+
| id | content | sendtime            |
+----+---------+---------------------+
|  1 | hello1  | 2023-05-04 17:41:42 |
|  2 | hello2  | 2023-05-04 17:41:47 |
+----+---------+---------------------+
2 rows in set (0.01 sec)

• 显示所有留言信息，发布日期只显示日期，不用显示时间

select content,date(sendtime) from msg;

mysql> select content,date(sendtime) from msg;
+---------+----------------+
| content | date(sendtime) |
+---------+----------------+
| hello1  | 2023-05-04     |
| hello2  | 2023-05-04     |
+---------+----------------+
2 rows in set (0.01 sec)

• 请查询在2分钟内发布的贴子

select * from msg where date_add(sendtime,interval 2 minute) > now();

理解：

------------------------------|-----------|-------------|------------------

                           初始时间     now()       初始时间+2min           

1.2字符串函数

charset(str)	返回字符串字符集
concat(string2 [,...])	连接字符串
instr(string,substring)	返回substring在string中出现的位置，没有返回0
ucase(string2)	转换成大写
lcase(string2)	转换成小写
left(string2,length)	从string2中的左边起取length个字符
length(string)	string的长度
replace(str,search_str,replace_str)	在str中用replace_str替换search_str
strcmp(string1,string2)	逐字符比较两字符的大小
substring(str,position [,length])	从str的postion开始，取length个字符
ltrim(string) rtrim(string) trim(string)	去除前空格或后空格

案例：

创建表

mysql> create table emp( 
	-> eid int primary key auto_increment, 
	-> ename varchar(20), 
	-> chinese int,
    -> math int,
    -> english int);
Query OK, 0 rows affected (0.23 sec)

插入数据若干：

mysql> insert into emp values(null,'孙悟空',80,90,77);
Query OK, 1 row affected (0.02 sec)

mysql> insert into emp values(null,'沙和尚',62,67,72);
Query OK, 1 row affected (0.03 sec)

• 获取emp表的ename列的字符集

mysql> select charset(ename) from emp;
+----------------+
| charset(ename) |
+----------------+
| utf8           |
| utf8           |
+----------------+
2 rows in set (0.01 sec)

• 要求显示emp表中的信息，显示格式为：xxx的语文成绩是xxx分，数学xxx分，英语xxx分

mysql> select concat(ename,'的语文成绩是',chinese,'分,数学',math,'分,英语',english,'分') as '分数' from emp;
+----------------------------------------------------------+
| 分数                                                     |
+----------------------------------------------------------+
| 孙悟空的语文成绩是80分,数学90分,英语77分                 |
| 沙和尚的语文成绩是62分,数学67分,英语72分                 |
+----------------------------------------------------------+
2 rows in set (0.01 sec)

• 求学生表中学生姓名占用的字节数

mysql> select length(ename),ename from emp;
+---------------+-----------+
| length(ename) | ename     |
+---------------+-----------+
|             9 | 孙悟空    |
|             9 | 沙和尚    |
+---------------+-----------+
2 rows in set (0.01 sec)

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，中文标识多个字节数(与字符集编码有关)

• 讲emp表中所有名字中有‘孙’的替换成’sun’

mysql> select replace(ename,'孙','sun'),ename from emp;
+----------------------------+-----------+
| replace(ename,'孙','sun')  | ename     |
+----------------------------+-----------+
| sun悟空                    | 孙悟空    |
| 沙和尚                     | 沙和尚    |
+----------------------------+-----------+
2 rows in set (0.01 sec)

• 截取emp表中ename字段的第二个到第三个字符

mysql> select substring(ename,2,2),ename from emp;
+----------------------+-----------+
| substring(ename,2,2) | ename     |
+----------------------+-----------+
| 悟空                 | 孙悟空    |
| 和尚                 | 沙和尚    |
+----------------------+-----------+
2 rows in set (0.01 sec)

注意：这里一个汉字算一个字符，和一个字符占几个字节无关。

• 以首字母小写的方式显示所有员工的姓名

mysql> select * from emp;
+-----+-----------+---------+------+---------+
| eid | ename     | chinese | math | english |
+-----+-----------+---------+------+---------+
|   1 | 孙悟空    |      80 |   90 |      77 |
|   2 | 沙和尚    |      62 |   67 |      72 |
|   3 | Lisa      |      70 |   80 |      90 |
|   4 | Jack      |      70 |   80 |      90 |
|   5 | Pony      |      70 |   80 |      90 |
|   6 | Tony      |      70 |   80 |      90 |
+-----+-----------+---------+------+---------+
6 rows in set (0.00 sec)

mysql> select concat(lcase(substring(ename, 1, 1)),substring(ename,2)) from emp;
+----------------------------------------------------------+
| concat(lcase(substring(ename, 1, 1)),substring(ename,2)) |
+----------------------------------------------------------+
| 孙悟空                                                   |
| 沙和尚                                                   |
| lisa                                                     |
| jack                                                     |
| pony                                                     |
| tony                                                     |
+----------------------------------------------------------+
6 rows in set (0.00 sec)

1.3数字函数

函数名称	描述
abs(number)	绝对值函数
bin(decimal_number)	十进制转换二进制
hex(decimalNumber)	转换成十六进制
conv(number,from_base,to_base)	进制转换
ceiling(number)	向上取整
floor(number)	向下取整
format(number,decimal_places)	格式化，保留小数位数
rand()	返回随机浮点数，范围[0.0,1.0)
mod(number,denominator)	取模，求余

• 绝对值

mysql> select abs(-100.2);
+-------------+
| abs(-100.2) |
+-------------+
|       100.2 |
+-------------+
1 row in set (0.01 sec)

• 向上取整

mysql> select ceiling(23.4);
+---------------+
| ceiling(23.4) |
+---------------+
|            24 |
+---------------+
1 row in set (0.00 sec)

mysql> select ceiling(-23.4);
+----------------+
| ceiling(-23.4) |
+----------------+
|            -23 |
+----------------+
1 row in set (0.00 sec)

• 向下取整

mysql> select floor(23.4);
+-------------+
| floor(23.4) |
+-------------+
|          23 |
+-------------+
1 row in set (0.00 sec)
mysql> select floor(-23.4);
+--------------+
| floor(-23.4) |
+--------------+
|          -24 |
+--------------+
1 row in set (0.00 sec)

• 保留2位小数位数(小数四舍五入)

mysql> select format(12.3456789,2);
+----------------------+
| format(12.3456789,2) |
+----------------------+
| 12.35                |
+----------------------+
1 row in set (0.00 sec)

• 产生随机数

mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.5188169232744881 |
+--------------------+
1 row in set (0.00 sec)

1.4其他函数

• user()查询当前用户

mysql> select user();
+--------+
| user() |
+--------+
| Lxy@   |
+--------+
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('123456');
+----------------------------------+
| md5('123456')                    |
+----------------------------------+
| e10adc3949ba59abbe56e057f20f883e |
+----------------------------------+
1 row in set (0.00 sec)

• database()显示当前正在使用的数据库

mysql> select database();
+----------------+
| database()     |
+----------------+
| 104_db_lesson7 |
+----------------+
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('Lxy');
+-------------------------------------------+
| password('Lxy')                           |
+-------------------------------------------+
| *48708BC40FD52498209A7778EDA8CB7BC76F8710 |
+-------------------------------------------+
1 row in set, 1 warning (0.00 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 (0.01 sec)

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

(本篇完)