【MySQL】7.0 内置函数

英语单词

interval 间隔

7. 函数

7.1 日期函数

函数名称	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中添加日期或时间interval后面的数值可以是:year minute second day
date_sub(date, 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
datediff(date1, date2)	两个日期的查，单位是天
now()	当前日期时间

# date 日期 datetime 日期加时间 timestamp 自动更新的日期和时间 

MariaDB [scott]> select current_time();
+----------------+
| current_time() |
+----------------+
| 11:54:05       |
+----------------+
1 row in set (0.00 sec)

MariaDB [scott]> select date_add('2022-12-30', interval 44 day);
+-----------------------------------------+
| date_add('2022-12-30', interval 44 day) |
+-----------------------------------------+
| 2023-02-12                              |
+-----------------------------------------+
1 row in set (0.00 sec)

# 当然我们也可以加上一个负数
MariaDB [scott]> select date_add('2022-12-30', interval -44 day);
+------------------------------------------+
| date_add('2022-12-30', interval -44 day) |
+------------------------------------------+
| 2022-11-16                               |
+------------------------------------------+
1 row in set (0.00 sec)

# 计算两个日期中间间隔的天数
MariaDB [scott]> select datediff('1949-10-1', '2022-12-30');
+-------------------------------------+
| datediff('1949-10-1', '2022-12-30') |
+-------------------------------------+
|                              -26753 |
+-------------------------------------+
1 row in set (0.00 sec)

接下来创建一个表演示一下

MariaDB [table_function]> create table if not exists tmp(
    -> id int unsigned primary key auto_increment, 
    -> birthday date not null);
Query OK, 0 rows affected (0.01 sec)

MariaDB [table_function]> desc tmp
    -> ;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| id       | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| birthday | date             | NO   |     | NULL    |                |
+----------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

# 我们既可以手动写入values的值
MariaDB [table_function]> insert into tmp (birthday) values ('1980-09-09');
Query OK, 1 row affected (0.00 sec)

#也可以使用函数的返回值
MariaDB [table_function]> insert into tmp (birthday) values ('current_date()');
Query OK, 1 row affected, 1 warning (0.00 sec)

MariaDB [table_function]> insert into tmp (birthday) values (current_date());
Query OK, 1 row affected (0.01 sec)

MariaDB [table_function]> select * from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 1980-09-09 |
|  2 | 0000-00-00 |   
|  3 | 2022-12-30 |
+----+------------+
3 rows in set (0.00 sec)
# 使用函数插入不需要加'' 不然就会变成2这样

# 构建一个信息列表
MariaDB [table_function]> create table msgl( id int unsigned primary key auto_increment, content varchar(30) not null, sendtime datetime comment '日期 + 时间' );
Query OK, 0 rows affected (0.02 sec)

MariaDB [table_function]> desc msgl;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| id       | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| content  | varchar(30)      | NO   |     | NULL    |                |
| sendtime | datetime         | YES  |     | NULL    |                |
+----------+------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

MariaDB [table_function]> insert into msgl (content, sendtime) values ('tw是中国不可分隔的一部分!', '2022-01-01 12:34:43');
Query OK, 1 row affected (0.00 sec)

MariaDB [table_function]> insert into msgl (content, sendtime) values ('tw是中国不可分隔!', now());
Query OK, 1 row affected (0.00 sec)

MariaDB [table_function]> select * from msgl;
+----+--------------------------------------+---------------------+
| id | content                              | sendtime            |
+----+--------------------------------------+---------------------+
|  1 | tw是中国不可分隔的一部分!            | 2022-01-01 12:34:43 |
|  2 | tw是中国不可分隔!                    | 2022-12-30 12:21:34 |
+----+--------------------------------------+---------------------+
2 rows in set (0.00 sec)

MariaDB [table_function]> create table if not exists msg2( id int unsigned primary key auto_increment, content varchar(40) not null, sendtime timestamp);
Query OK, 0 rows affected (0.01 sec)

MariaDB [table_function]> desc msg2;
+----------+------------------+------+-----+-------------------+-----------------------------+
| Field    | Type             | Null | Key | Default           | Extra                       |
+----------+------------------+------+-----+-------------------+-----------------------------+
| id       | int(10) unsigned | NO   | PRI | NULL              | auto_increment              |
| content  | varchar(40)      | NO   |     | NULL              |                             |
| sendtime | timestamp        | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+----------+------------------+------+-----+-------------------+-----------------------------+
3 rows in set (0.00 sec)

MariaDB [table_function]> insert into msg2 (content) values ('first message');
Query OK, 1 row affected (0.00 sec)
# timestamp类型会给我们自动添加时间日期信息
MariaDB [table_function]> select * from msg2;
+----+---------------+---------------------+
| id | content       | sendtime            |
+----+---------------+---------------------+
|  1 | first message | 2022-12-30 14:33:47 |
+----+---------------+---------------------+
1 row in set (0.00 sec)

查询两分钟内发布的贴子

MariaDB [table_function]> insert into msg2 (content) values ('third message');
Query OK, 1 row affected (0.01 sec)

MariaDB [table_function]> select * from msg2 where date_add(sendtime, interval 2 minute) >= now();
+----+---------------+---------------------+
| id | content       | sendtime            |
+----+---------------+---------------------+
|  3 | third message | 2022-12-30 14:40:09 |
+----+---------------+---------------------+
1 row in set (0.00 sec)

7.2字符串函数

函数	描述
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中出现的未知
ucase(string2)	转换大写
lcase(string2)	转换成小写
left(string2, length)	从string2中左边起取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)	逐个字符比较字符串大小
substring(str, position[, length])	从str的postion开始，取length个字符
ltrim(string) rtrim(string) trim(string)	取出前空格和后空格

案例：

• 获取msg2表中content字段的字符集

  MariaDB [table_function]> select charset(content) from msg2;
  +------------------+
  | charset(content) |
  +------------------+
  | utf8             |
  | utf8             |
  | utf8             |
  +------------------+
  3 rows in set (0.00 sec)
  
  

• 拼接字符串

  MariaDB [table_function]> select concat('你好', '世界') msg;
  +--------------+
  | msg          |
  +--------------+
  | 你好世界     |
  +--------------+
  1 row in set (0.00 sec)
  
  

• 求学生表中学生姓名占用的字节数

  select length(name) from students;
  

• 替换字符串

  select replace('hello world', 'world', 'linux');
  +------------------------------------------+
  | replace('hello world', 'world', 'linux') |
  +------------------------------------------+
  | hello linux                              |
  +------------------------------------------+
  1 row in set (0.00 sec)
  

• 以首字母小写的方式显示所有员工的姓名

  select concat(lcase(substring(ename,1,1)), substring(ename,2)) from EMP;
  

7.3数学函数

函数	描述
abs(number)	绝对值函数
bin(decimal_number)	十进制转换二进制
hex(decimalNumber)	十进制转换十六进制
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
hex(decimalNumber)	转化成十六进制
rand()	返回随机浮点数,范围[0.0. 1.0）
mod(number,denominator)	取模，求余

7.4其它函数

• 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

• database()显示当前正在使用的数据库

• password() MySQL使用这个函数对数据库进行加密

• ifnull(val1, val2) 如果val1 == null返回val2否则返回val1