【MySQL】内置函数

雾里看山2025-02-22 16:59

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本篇主题 ：【MySQL】内置函数
发布时间 ：2025.2.20
隶属专栏 ：MySQL

目录

日期函数

函数介绍

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 中添加日期或者时间 interval 后的数值单位可以是：year minute second day
date_sub(date, interval d_value_type) 在 date 中减去日期或者时间 interval 后的数值单位可以是：year minute second day
datediff(date1,date2) 两个日期的差，单位是天
now() 当前日期时间

函数使用

current_date() 获得年月日

sql 复制代码 
mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2025-02-18     |
+----------------+
1 row in set (0.02 sec)

current_time() 获得时分秒

sql 复制代码 
mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 15:33:32       |
+----------------+
1 row in set (0.00 sec)

current_timestamp() 获得时间戳

sql 复制代码 
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2025-02-18 15:33:37 |
+---------------------+
1 row in set (0.00 sec)

now() 获取日期时间

sql 复制代码 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2025-02-18 15:36:23 |
+---------------------+
1 row in set (0.00 sec)

date 截取日期

参数是具体时间

sql 复制代码 
mysql> select date('1949-10-01 00:00:00');
+-----------------------------+
| date('1949-10-01 00:00:00') |
+-----------------------------+
| 1949-10-01                  |
+-----------------------------+
1 row in set (0.00 sec)

参数是时间函数

sql 复制代码 
mysql> select date(now());
+-------------+
| date(now()) |
+-------------+
| 2025-02-18  |
+-------------+
1 row in set (0.00 sec)

date_add 日期加

具体日期加时间

sql 复制代码 
mysql> select date_add('2050-01-01', interval 10 day);
+-----------------------------------------+
| date_add('2050-01-01', interval 10 day) |
+-----------------------------------------+
| 2050-01-11                              |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> select date_add('2050-01-01', interval 10 month);
+-------------------------------------------+
| date_add('2050-01-01', interval 10 month) |
+-------------------------------------------+
| 2050-11-01                                |
+-------------------------------------------+
1 row in set (0.00 sec)

函数日期加时间

sql 复制代码 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2025-02-18 15:47:57 |
+---------------------+
1 row in set (0.00 sec)

mysql> select date_add(now(), interval 10 month);
+------------------------------------+
| date_add(now(), interval 10 month) |
+------------------------------------+
| 2025-12-18 15:47:59                |
+------------------------------------+
1 row in set (0.00 sec)

date_sub 日期减

具体日期减时间

sql 复制代码 
mysql> select date_sub('2050-01-01', interval 10 day);
+-----------------------------------------+
| date_sub('2050-01-01', interval 10 day) |
+-----------------------------------------+
| 2049-12-22                              |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> select date_sub('2050-01-01', interval 10 month);
+-------------------------------------------+
| date_sub('2050-01-01', interval 10 month) |
+-------------------------------------------+
| 2049-03-01                                |
+-------------------------------------------+
1 row in set (0.00 sec)

函数日期加时间

sql 复制代码 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2025-02-18 15:49:50 |
+---------------------+
1 row in set (0.00 sec)

mysql> select date_sub(now(), interval 10 month);
+------------------------------------+
| date_sub(now(), interval 10 month) |
+------------------------------------+
| 2024-04-18 15:50:00                |
+------------------------------------+
1 row in set (0.00 sec)

datediff 日期差

sql 复制代码 
mysql> select datediff('2020-10-23', '2013-04-13');
+--------------------------------------+
| datediff('2020-10-23', '2013-04-13') |
+--------------------------------------+
|                                 2750 |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> select datediff(date(now()), '1949-10-01');
+-------------------------------------+
| datediff(date(now()), '1949-10-01') |
+-------------------------------------+
|                               27534 |
+-------------------------------------+
1 row in set (0.00 sec)

案例

案例1 ：创建一张表，记录生日

sql 复制代码 
mysql> create table tmp(
    -> id bigint primary key auto_increment,
    -> birthday date
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> desc tmp;
+----------+------------+------+-----+---------+----------------+
| Field    | Type       | Null | Key | Default | Extra          |
+----------+------------+------+-----+---------+----------------+
| id       | bigint(20) | NO   | PRI | NULL    | auto_increment |
| birthday | date       | YES  |     | NULL    |                |
+----------+------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

插入数据

sql 复制代码 
mysql> insert into tmp (brithday) values ('1990-02-24');
ERROR 1054 (42S22): Unknown column 'brithday' in 'field list'
mysql> insert into tmp (birthday) values ('1990-02-24');
Query OK, 1 row affected (0.00 sec)

mysql> insert into tmp (birthday) values ('1980-03-05');
Query OK, 1 row affected (0.01 sec)

mysql> insert into tmp (birthday) values (current_date());
Query OK, 1 row affected (0.00 sec)

mysql> select * from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 1990-02-24 |
|  2 | 1980-03-05 |
|  3 | 2025-02-18 |
+----+------------+
3 rows in set (0.00 sec)

案例二：创建一个留言表

sql 复制代码 
mysql> create table msg(
    -> id bigint primary key auto_increment,
    -> content varchar(30) not null,
    -> sendtime datetime
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> desc msg;
+----------+-------------+------+-----+---------+----------------+
| Field    | Type        | Null | Key | Default | Extra          |
+----------+-------------+------+-----+---------+----------------+
| id       | bigint(20)  | NO   | PRI | NULL    | auto_increment |
| content  | varchar(30) | NO   |     | NULL    |                |
| sendtime | datetime    | YES  |     | NULL    |                |
+----------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

插入数据

sql 复制代码 
mysql> insert into msg (content, sendtime) values ('纸上得来终觉浅，绝知此事要躬行', now());
Query OK, 1 row affected (0.00 sec)

mysql> insert into msg (content, sendtime) values ('不敢高声语，恐惊天上人', now());
Query OK, 1 row affected (0.00 sec)

mysql> select * from msg;
+----+-----------------------------------------------+---------------------+
| id | content                                       | sendtime            |
+----+-----------------------------------------------+---------------------+
|  1 | 纸上得来终觉浅，绝知此事要躬行                     | 2025-02-18 16:27:09 | 
|  2 | 不敢高声语，恐惊天上人                            | 2025-02-18 16:28:51 |
+----+-----------------------------------------------+---------------------+
2 rows in set (0.00 sec)

请查询在2分钟内发布的帖子
select * from msg where date_add(sendtime, interval 2 minute) > now();

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

sql 复制代码 
mysql> insert into msg (content, sendtime) values ('举头望明月，低头思故乡', now());
Query OK, 1 row affected (0.00 sec)

mysql> select * from msg where date_add(sendtime, interval 2 minute) > now();
+----+-----------------------------------+---------------------+
| id | content                           | sendtime            |
+----+-----------------------------------+---------------------+
|  3 | 举头望明月，低头思故乡                | 2025-02-18 16:44:15 |
+----+-----------------------------------+---------------------+
1 row in set (0.00 sec)

mysql> select * from msg where sendtime > date_sub(now(), interval 2 minute);
+----+-----------------------------------+---------------------+
| id | content                           | sendtime            |
+----+-----------------------------------+---------------------+
|  3 | 举头望明月，低头思故乡                | 2025-02-18 16:44:15 |
+----+-----------------------------------+---------------------+
1 row in set (0.00 sec)

字符串函数

函数介绍

函数 描述
charset(str) 返回字符串字符集
concat(string2 [, ...]) 连接字符串
instr(string,sunstring) 返回substring在string中出现的位置
ucase(string2) 转换成大写
lcase(string2) 转换成小写
left(string2,length) 从string2的左边取length个字符
length(string) string的长度
replace(str, search_str,replace_str) 在str中用replace_str代替search_str
strcmp(string1,string2) 逐字符比较两字符串大小
substring(str, position [, length]) 从str的position开始，取length个字符
ltrim(string) rtrim(string) trim(string) 去除前空格或者后空格

函数使用

charset 查字符集

应用场景：当插入时有的地方乱码了，可以查看字符集是否有问题。

sql 复制代码 
mysql> select charset('1234');
+-----------------+
| charset('1234') |
+-----------------+
| utf8            |
+-----------------+
1 row in set (0.00 sec)

mysql> select charset(sendtime) from msg;
+-------------------+
| charset(sendtime) |
+-------------------+
| binary            |
| binary            |
| binary            |
+-------------------+
3 rows in set (0.00 sec)

mysql> select charset(content) from msg;
+------------------+
| charset(content) |
+------------------+
| utf8             |
| utf8             |
| utf8             |
+------------------+
3 rows in set (0.00 sec)

concat 拼接字符串

sql 复制代码 
mysql> select concat('a', 'b', 'c', 123, 3.14) as res;
+------------+
| res        |
+------------+
| abc1233.14 |
+------------+
1 row in set (0.00 sec)

instr 查找子串

sql 复制代码 
mysql> select instr('abcd1234efg','1234');
+-----------------------------+
| instr('abcd1234efg','1234') |
+-----------------------------+
|                           5 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> select instr('abcd1234efg','1234a');
+------------------------------+
| instr('abcd1234efg','1234a') |
+------------------------------+
|                            0 |
+------------------------------+
1 row in set (0.00 sec)

ucase 转大写

sql 复制代码 
mysql> select ucase('abcd1234ABCD');
+-----------------------+
| ucase('abcd1234ABCD') |
+-----------------------+
| ABCD1234ABCD          |
+-----------------------+
1 row in set (0.00 sec)

lcase 转小写

sql 复制代码 
mysql> select lcase('abcd1234ABCD');
+-----------------------+
| lcase('abcd1234ABCD') |
+-----------------------+
| abcd1234abcd          |
+-----------------------+
1 row in set (0.00 sec)

left 从左取length个字符

sql 复制代码 
mysql> select left('abcd1234',5);
+--------------------+
| left('abcd1234',5) |
+--------------------+
| abcd1              |
+--------------------+
1 row in set (0.00 sec)

right 从右取length个字符

sql 复制代码 
mysql> select right('abcd1234',5);
+---------------------+
| right('abcd1234',5) |
+---------------------+
| d1234               |
+---------------------+
1 row in set (0.00 sec)

length 字符串占的字节数

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）

sql 复制代码 
mysql> select length('abcd1234');
+--------------------+
| length('abcd1234') |
+--------------------+
|                  8 |
+--------------------+
1 row in set (0.00 sec)

ltrim , rtrim, trim

sql 复制代码 
mysql> select ltrim('        你好             ') as res;
+---------------------+
| res                 |
+---------------------+
| 你好                |
+---------------------+
1 row in set (0.00 sec)

mysql> select rtrim('        你好             ') as res;
+----------------+
| res            |
+----------------+
|         你好   |
+----------------+
1 row in set (0.01 sec)

mysql> select trim('        你好             ') as res;
+--------+
| res    |
+--------+
| 你好   |
+--------+
1 row in set (0.00 sec)

案例

  1. 获取emp表的ename列的字符集
sql 复制代码 
mysql> select charset(ename) from emp;
+----------------+
| charset(ename) |
+----------------+
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
+----------------+
14 rows in set (0.00 sec)
  1. 要求显示exam_result表中的信息，显示格式："考生姓名：XXX,总分：XXX,语文成绩：XXX,数学成绩XXX,英语成绩XXX"
sql 复制代码 
mysql> select concat('考生姓名：', name, ',总分：', chinese+math+english , ',语文成绩：',chinese,',数学成绩',math,',英语成绩', english) msg from exam_result;
+----------------------------------------------------------------------------------------+
| msg                                                                                    |
+----------------------------------------------------------------------------------------+
| 考生姓名：唐三藏,总分：288,语文成绩：134,数学成绩98,英语成绩56                                 |
| 考生姓名：猪悟能,总分：364,语文成绩：176,数学成绩98,英语成绩90                                 |
| 考生姓名：曹孟德,总分：297,语文成绩：140,数学成绩90,英语成绩67                                 |
| 考生姓名：孙权,总分：291,语文成绩：140,数学成绩73,英语成绩78                                   |
| 考生姓名：宋公明,总分：275,语文成绩：150,数学成绩95,英语成绩30                                 |
+----------------------------------------------------------------------------------------+
5 rows in set (0.00 sec)
  1. 求学生表中学生姓名占用的字节数
sql 复制代码 
mysql> select name, length(name) from exam_result;
+-----------+--------------+
| name      | length(name) |
+-----------+--------------+
| 唐三藏    |            9 |
| 猪悟能    |            9 |
| 曹孟德    |            9 |
| 孙权      |            6 |
| 宋公明    |            9 |
+-----------+--------------+
5 rows in set (0.00 sec)
  1. 将EMP表中所有名字中有S的替换成'上海'
sql 复制代码 
mysql> select ename, replace(ename, 'S', '上海') from emp;
+--------+-------------------------------+
| ename  | replace(ename, 'S', '上海')   |
+--------+-------------------------------+
| SMITH  | 上海MITH                      |
| ALLEN  | ALLEN                         |
| WARD   | WARD                          |
| JONES  | JONE上海                      |
| MARTIN | MARTIN                        |
| BLAKE  | BLAKE                         |
| CLARK  | CLARK                         |
| SCOTT  | 上海COTT                      |
| KING   | KING                          |
| TURNER | TURNER                        |
| ADAMS  | ADAM上海                      |
| JAMES  | JAME上海                      |
| FORD   | FORD                          |
| MILLER | MILLER                        |
+--------+-------------------------------+
14 rows in set (0.00 sec)
  1. 截取EMP表中ename字段的第二个到第三个字符
sql 复制代码 
mysql> select substring(ename, 2, 2) ename from emp;
+-------+
| ename |
+-------+
| MI    |
| LL    |
| AR    |
| ON    |
| AR    |
| LA    |
| LA    |
| CO    |
| IN    |
| UR    |
| DA    |
| AM    |
| OR    |
| IL    |
+-------+
14 rows in set (0.00 sec)
  1. 以首字母小写的方式显示所有员工的姓名
sql 复制代码 
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)

数学函数

函数介绍

函数 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
rand() 返回随机浮点数，范围[0.0, 1.0)
mod（number, denominator） 取模，求余

函数使用

绝对值

sql 复制代码 
mysql> select abs(-10);
+----------+
| abs(-10) |
+----------+
|       10 |
+----------+
1 row in set (0.00 sec)

mysql> select abs(10);
+---------+
| abs(10) |
+---------+
|      10 |
+---------+
1 row in set (0.00 sec)

mysql> select abs(-10.23);
+-------------+
| abs(-10.23) |
+-------------+
|       10.23 |
+-------------+
1 row in set (0.00 sec)

十进制转换二进制

sql 复制代码 
mysql> select bin(5);
+--------+
| bin(5) |
+--------+
| 101    |
+--------+
1 row in set (0.00 sec)

十进制转换十六进制

sql 复制代码 
mysql> select hex(16);
+---------+
| hex(16) |
+---------+
| 10      |
+---------+
1 row in set (0.00 sec)

进制转换

sql 复制代码 
mysql> select conv(20, 10 ,16);
+------------------+
| conv(20, 10 ,16) |
+------------------+
| 14               |
+------------------+
1 row in set (0.00 sec)

向上取整

sql 复制代码 
mysql> select ceiling(2.001);
+----------------+
| ceiling(2.001) |
+----------------+
|              3 |
+----------------+
1 row in set (0.00 sec)

mysql> select ceiling(-2.99);
+----------------+
| ceiling(-2.99) |
+----------------+
|             -2 |
+----------------+
1 row in set (0.00 sec)

向下取整

sql 复制代码 
mysql> select floor(-2.55);
+--------------+
| floor(-2.55) |
+--------------+
|           -3 |
+--------------+
1 row in set (0.00 sec)

mysql> select floor(2.55);
+-------------+
| floor(2.55) |
+-------------+
|           2 |
+-------------+
1 row in set (0.00 sec)

格式化，保留小数位数

会四舍五入

sql 复制代码 
mysql> select format(3.1415926, 3);
+----------------------+
| format(3.1415926, 3) |
+----------------------+
| 3.142                |
+----------------------+
1 row in set (0.00 sec)

生成随机数

sql 复制代码 
mysql> select format(rand()*100, 0);
+-----------------------+
| format(rand()*100, 0) |
+-----------------------+
| 77                    |
+-----------------------+
1 row in set (0.00 sec)

取模，求余

sql 复制代码 
mysql> select mod(10,3);
+-----------+
| mod(10,3) |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql> select mod(10,-3);
+------------+
| mod(10,-3) |
+------------+
|          1 |
+------------+
1 row in set (0.00 sec)

mysql> select mod(-10,-3);
+-------------+
| mod(-10,-3) |
+-------------+
|          -1 |
+-------------+
1 row in set (0.00 sec)

mysql> select mod(-10,3);
+------------+
| mod(-10,3) |
+------------+
|         -1 |
+------------+
1 row in set (0.00 sec)

其他函数

查看当前用户

sql 复制代码 
mysql> select user();
+--------+
| user() |
+--------+
| root@  |
+--------+
1 row in set (0.00 sec)

查看所在数据库

sql 复制代码 
mysql> select database();
+------------+
| database() |
+------------+
| scott      |
+------------+
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

sql 复制代码 
mysql> select md5('afe152');
+----------------------------------+
| md5('afe152')                    |
+----------------------------------+
| cf3c4ad0dc7525587fc64a9f60d6f8e0 |
+----------------------------------+
1 row in set (0.01 sec)

password()

MySQL数据库使用该函数对用户加密

sql 复制代码 
mysql> select password('afe152');
+-------------------------------------------+
| password('afe152')                        |
+-------------------------------------------+
| *B9CDC4309CB123945F0951B63E13118A9235A4E0 |
+-------------------------------------------+
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

sql 复制代码 
mysql> select ifnull(20, 10);
+----------------+
| ifnull(20, 10) |
+----------------+
|             20 |
+----------------+
1 row in set (0.00 sec)

mysql> select ifnull(null, 10);
+------------------+
| ifnull(null, 10) |
+------------------+
|               10 |
+------------------+
1 row in set (0.00 sec)

mysql> select ifnull(20, null);
+------------------+
| ifnull(20, null) |
+------------------+
|               20 |
+------------------+
1 row in set (0.00 sec)

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

⚠️ 写在最后：以上内容是我在学习以后得一些总结和概括，如有错误或者需要补充的地方欢迎各位大佬评论或者私信我交流！！！

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