【MySQL学习笔记】（七）内置函数

日期函数

示例

• 获得当前年月日

mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2023-09-03     |
+----------------+
1 row in set (0.00 sec)

• 获得当前时分秒

mysql> select current_time;
+--------------+
| current_time |
+--------------+
| 09:20:15     |
+--------------+
1 row in set (0.00 sec)

• 获得当前时间戳

mysql中该函数会自动将时间戳转化为年月日时分秒的格式

mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2023-09-03 09:20:49 |
+---------------------+
1 row in set (0.00 sec)

• date(datetime),返回 datetime 参数中的日期部分（年月日部分）

mysql> select date(current_date());
+----------------------+
| date(current_date()) |
+----------------------+
| 2023-09-03           |
+----------------------+
1 row in set (0.00 sec)//虽然currnt_time()获得的只有时分秒时间，但该函数内部获得时间的时候是可以获得日期（年月日）的
mysql> select date(current_time());
+----------------------+
| date(current_time()) |
+----------------------+
| 2023-09-03           |
+----------------------+
1 row in set (0.00 sec)mysql> select date(current_timestamp());
+---------------------------+
| date(current_timestamp()) |
+---------------------------+
| 2023-09-03                |
+---------------------------+
1 row in set (0.00 sec)

• 在日期的基础上加上日期

//在 2023-9-3 的基础上加上 50 天
mysql> select date_add('2023-9-3',interval 50 day);
+--------------------------------------+
| date_add('2023-9-3',interval 50 day) |
+--------------------------------------+
| 2023-10-23                           |
+--------------------------------------+
1 row in set (0.00 sec)

• 在日期的基础上减去日期

//在 2023-10-23 的基础上减去 50 天
mysql> select date_sub('2023-10-23',interval 50 day);
+----------------------------------------+
| date_sub('2023-10-23',interval 50 day) |
+----------------------------------------+
| 2023-09-03                             |
+----------------------------------------+
1 row in set (0.00 sec)

• 计算两个日期相差多少天

mysql> select datediff('2023-9-3','2023-10-23');
+-----------------------------------+
| datediff('2023-9-3','2023-10-23') |
+-----------------------------------+
|                               -50 |
+-----------------------------------+
1 row in set (0.00 sec)

案例-1

• 创建一张表，记录生日

mysql> create table tmp (id int primary key auto_increment,birthday date not null);
Query OK, 0 rows affected (0.01 sec)mysql> desc tmp;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| birthday | date    | NO   |     | NULL    |                |
+----------+---------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

• 添加当前日期

mysql> insert into tmp(birthday) values(current_date());
Query OK, 1 row affected (0.00 sec)mysql> select* from tmp;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 2023-09-03 |
+----+------------+
1 row in set (0.00 sec)

案例-2

• 创建一个留言表

mysql> create table msg(id int primary key auto_increment,content varchar(30) not null,sendtime datetime);
Query OK, 0 rows affected (0.01 sec)mysql> desc msg;
+----------+-------------+------+-----+---------+----------------+
| Field    | Type        | Null | Key | Default | Extra          |
+----------+-------------+------+-----+---------+----------------+
| id       | int(11)     | 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

mysql> insert into msg(content,sendtime) values('hello',now());
Query OK, 1 row affected (0.00 sec)mysql> insert into msg(content,sendtime) values('hi',now());
Query OK, 1 row affected (0.00 sec)mysql> select* from msg;
+----+---------+---------------------+
| id | content | sendtime            |
+----+---------+---------------------+
|  1 | hello   | 2023-09-03 09:48:45 |
|  2 | hi      | 2023-09-03 09:48:52 |
+----+---------+---------------------+
2 rows in set (0.00 sec)

• 显示所有留言信息，发布日期只显示到年月日，不用显示具体时间

mysql> select content ,date(sendtime) from msg;
+---------+----------------+
| content | date(sendtime) |
+---------+----------------+
| hello   | 2023-09-03     |
| hi      | 2023-09-03     |
+---------+----------------+
2 rows in set (0.00 sec)

• 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 | hello   | 2023-09-03 09:48:45 |
|  2 | hi      | 2023-09-03 09:48:52 |
+----+---------+---------------------+
2 rows in set (0.00 sec)

理解：------------------------------|-----------|-------------|------------------初始时间     now()       初始时间+10min  

字符串函数

示例

• 获取表中的某一列的字符集

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

• 字符串连接

mysql> select concat('app','le');
+--------------------+
| concat('app','le') |
+--------------------+
| apple              |
+--------------------+
1 row in set (0.00 sec)

• 某字符串中是否包含某子串，包含则返回出现的位置，未包含则返回 0

mysql> select instr('apple','ple');
+----------------------+
| instr('apple','ple') |
+----------------------+
|                    3 |
+----------------------+
1 row in set (0.00 sec)mysql> select instr('apple','b');
+--------------------+
| instr('apple','b') |
+--------------------+
|                  0 |
+--------------------+
1 row in set (0.00 sec)

• 转换成大写

mysql> select ucase('abcA');
+---------------+
| ucase('abcA') |
+---------------+
| ABCA          |
+---------------+
1 row in set (0.00 sec)

• 转换成小写

mysql> select lcase('abcA');
+---------------+
| lcase('abcA') |
+---------------+
| abca          |
+---------------+
1 row in set (0.00 sec)

• 从字符串的左边或者右边起取 length 个字符

mysql> select left('abcdef',2);
+------------------+
| left('abcdef',2) |
+------------------+
| ab               |
+------------------+
1 row in set (0.00 sec)mysql> select right('abcdef',2);
+-------------------+
| right('abcdef',2) |
+-------------------+
| ef                |
+-------------------+
1 row in set (0.00 sec)

• 获取字符串的长度

mysql> select length('abc');
+---------------+
| length('abc') |
+---------------+
|             3 |
+---------------+
1 row in set (0.00 sec)mysql> select length('你好');
+------------------+
| length('你好')   |
+------------------+
|                6 |
+------------------+
1 row in set (0.00 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，中文表示多个字节数
（与字符集编码有关）

• 在字符串中用某子串替换掉其中的子串

//在 ‘hello world’ 中用 ‘jack’ 替换掉 ‘world’
mysql> select replace('hello world','world','jack');
+---------------------------------------+
| replace('hello world','world','jack') |
+---------------------------------------+
| hello jack                            |
+---------------------------------------+
1 row in set (0.00 sec)

• 逐字符比较字符串的大小

//第一个字符串小于第二个字符串返回 -1
mysql> select strcmp('abc','abd');
+---------------------+
| strcmp('abc','abd') |
+---------------------+
|                  -1 |
+---------------------+
1 row in set (0.00 sec)//相等返回 0
mysql> select strcmp('abc','abc');
+---------------------+
| strcmp('abc','abc') |
+---------------------+
|                   0 |
+---------------------+
1 row in set (0.00 sec)//大于返回 1
mysql> select strcmp('abd','abc');
+---------------------+
| strcmp('abd','abc') |
+---------------------+
|                   1 |
+---------------------+
1 row in set (0.00 sec)

• 从字符串的第 position 开始，取 length 个字符

//从第一个位置开始取三个字符
mysql> select substring('abcdefg','1',3);
+----------------------------+
| substring('abcdefg','1',3) |
+----------------------------+
| abc                        |
+----------------------------+
1 row in set (0.00 sec)

• 去除 前空格 / 后空格 / 前后空格

mysql> select '   hello   ' as res;
+-------------+
| res         |
+-------------+
|    hello    |
+-------------+
1 row in set (0.00 sec)mysql> select ltrim('   hello   ') as res;//去除前空格
+----------+
| res      |
+----------+
| hello    |
+----------+
1 row in set (0.00 sec)mysql> select rtrim('   hello   ') as res;//去除后空格
+----------+
| res      |
+----------+
|    hello |
+----------+
1 row in set (0.00 sec)mysql> select trim('   hello   ') as res; //去除前后空格
+-------+
| res   |
+-------+
| hello |
+-------+
1 row in set (0.00 sec)

数学函数

• 绝对值

mysql> select abs(-100.5);
+-------------+
| abs(-100.5) |
+-------------+
|       100.5 |
+-------------+
1 row in set (0.00 sec)

• 向上取整

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

• 向下取整

mysql> select floor(24.7);
+-------------+
| floor(24.7) |
+-------------+
|          24 |
+-------------+
1 row in set (0.00 sec)

• 保留小数位数

//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()             |
+--------------------+
| 0.2013088168588549 |
+--------------------+
1 row in set (0.00 sec)mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.2613807602425858 |
+--------------------+
1 row in set (0.00 sec)

其他函数

• 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()函数，MySQL数据库使用该函数对用户加密
• ifnull（val1， val2） 如果val1为null，返回val2，否则返回val1的值