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中减去日期或时间 interval后的数值单位可以是：year minute second day |
| datediff(datel,date2) | 两个去日期的差，单位是天 |
| now() | 当前日期时间 |
-- 年月日
mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2024-11-08 |
+----------------+
1 row in set (0.00 sec)-- 时分秒
mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 17:54:51 |
+----------------+
1 row in set (0.00 sec)-- 时间戳
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2024-11-08 17:55:00 |
+---------------------+
1 row in set (0.00 sec)-- 日期加日期
mysql> select date_add('2024-11-8',interval 10 day);
+---------------------------------------+
| date_add('2024-11-8',interval 10 day) |
+---------------------------------------+
| 2024-11-18 |
+---------------------------------------+
1 row in set (0.00 sec)-- 日期减日期
mysql> select date_sub('2024-11-8',interval 10 day);
+---------------------------------------+
| date_sub('2024-11-8',interval 10 day) |
+---------------------------------------+
| 2024-10-29 |
+---------------------------------------+
1 row in set (0.00 sec)-- 相差
mysql> select datediff('2024-11-8','2023-11-18');
+------------------------------------+
| datediff('2024-11-8','2023-11-18') |
+------------------------------------+
| 356 |
+------------------------------------+
1 row in set (0.00 sec)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) | 逐字符比较两字符串大小 |
| 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 |
3.数学函数
| 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) | 向上去整 |
| floor(number) | 向下去整 |
| format (number,decimal_places) | 格式化，保留小数位数 |
| hex(decimalNumber) | 转换成十六进制 |
| rand() | 返回随机浮点数，范围[0.0,1.0) |
| mod(number,denominator) | 取模，求余 |
-- 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)-- 保留两位小数位数，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() |
+---------------------+
| 0.41486722269548776 |
+---------------------+
1 row in set (0.00 sec)
