MySQL之函数详解
了解编程的人一般都会知道函数的重要性，丰富的函数有的时候可以给我们带来事半功倍的效果，在MySQL中提供了许多的内置函数，能够帮助开发人员编写简单快捷的SQL语句,除了这些内置的函数之外，用户也可以自定义函数，本次博客整理了一下MySQL中常用的函数，简单的介绍了自定函数，MySQL版本mysql-5.7.19。
常用函数
1.字符串函数
字符串函数是最常用的一种函数。下表列出了常用的字符串函数：
2.数值函数
MySQL中另外一类就是数值函数了。这些函数可以处理很多数值方面的运算，常见的数值运算函数如下：
3.日期和时间函数
有的时候们需要了解当前的时间，这时候我们就可以调用时间函数了。下面就是一些常用的时间函数：
下面来说一下DATE_FORMAT(date,format)：format可以有以下格式符：
Specifier Description %a Abbreviated weekday name (Sun..Sat) %b Abbreviated month name (Jan..Dec) %c Month, numeric (0..12) %D Day of the month with English suffix (0th, 1st, 2nd, 3rd, …) %d Day of the month, numeric (00..31) %e Day of the month, numeric (0..31) %f Microseconds (000000..999999) %H Hour (00..23) %h Hour (01..12) %I Hour (01..12) %i Minutes, numeric (00..59) %j Day of year (001..366) %k Hour (0..23) %l Hour (1..12) %M Month name (January..December) %m Month, numeric (00..12) %p AM or PM %r Time, 12-hour (hh:mm:ss followed by AM or PM) %S Seconds (00..59) %s Seconds (00..59) %T Time, 24-hour (hh:mm:ss) %U Week (00..53), where Sunday is the first day of the week; WEEK() mode 0 %u Week (00..53), where Monday is the first day of the week; WEEK() mode 1 %V Week (01..53), where Sunday is the first day of the week; WEEK() mode 2; used with %X %v Week (01..53), where Monday is the first day of the week; WEEK() mode 3; used with %x %W Weekday name (Sunday..Saturday) %w Day of the week (0=Sunday..6=Saturday) %X Year for the week where Sunday is the first day of the week, numeric, four digits; used with %V %x Year for the week, where Monday is the first day of the week, numeric, four digits; used with %v %Y Year, numeric, four digits %y Year, numeric (two digits) %% A literal % character %x x, for any “x” not listed above格式符
举个例子吧！
mysql> SELECT DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y');
+------------------------------------------------+
| DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y') |
+------------------------------------------------+
| Sunday October 2009 |
+------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT DATE_FORMAT(now(),'%H %k %I %r %T %S %w');
+-------------------------------------------+
| DATE_FORMAT(now(),'%H %k %I %r %T %S %w') |
+-------------------------------------------+
| 23 23 11 11:49:57 PM 23:49:57 57 5 |
+-------------------------------------------+
1 row in set (0.00 sec)再来看一下DATE_ADD(date,INTERVAL expr unit)：其中INTERVAL是关键字，expr是一个表达式，unit为间隔类型，MySQL提供如下的间隔类型:
unit Value Expected expr Format MICROSECOND MICROSECONDS SECOND SECONDS MINUTE MINUTES HOUR HOURS DAY DAYS WEEK WEEKS MONTH MONTHS QUARTER QUARTERS YEAR YEARS SECOND_MICROSECOND 'SECONDS.MICROSECONDS' MINUTE_MICROSECOND 'MINUTES:SECONDS.MICROSECONDS' MINUTE_SECOND 'MINUTES:SECONDS' HOUR_MICROSECOND 'HOURS:MINUTES:SECONDS.MICROSECONDS' HOUR_SECOND 'HOURS:MINUTES:SECONDS' HOUR_MINUTE 'HOURS:MINUTES' DAY_MICROSECOND 'DAYS HOURS:MINUTES:SECONDS.MICROSECONDS' DAY_SECOND 'DAYS HOURS:MINUTES:SECONDS' DAY_MINUTE 'DAYS HOURS:MINUTES' DAY_HOUR 'DAYS HOURS' YEAR_MONTH 'YEARS-MONTHS'间隔类型
举个例子吧！
mysql> SELECT DATE_ADD('2000-12-31 23:59:59',INTERVAL 1 SECOND);
+---------------------------------------------------+
| DATE_ADD('2000-12-31 23:59:59',INTERVAL 1 SECOND) |
+---------------------------------------------------+
| 2001-01-01 00:00:00 |
+---------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT DATE_ADD('2100-12-31 23:59:59',INTERVAL '1:1' MINUTE_SECOND);
+--------------------------------------------------------------+
| DATE_ADD('2100-12-31 23:59:59',INTERVAL '1:1' MINUTE_SECOND) |
+--------------------------------------------------------------+
| 2101-01-01 00:01:00 |
+--------------------------------------------------------------+
1 row in set (0.00 sec) 4.流程函数
流程函数也是很常用的一类函数，用户可以使用这类函数在一个SQL语句中实现条件选择，这样做能够提高语句的效率。
举例：
现在有员工工资表如下：
mysql> select * from salary; +----+---------+ | id | salary | +----+---------+ | 1 | 1000.00 | | 2 | 2000.00 | | 3 | 3000.00 | | 4 | 4000.00 | | 5 | 5000.00 | | 6 | 6000.00 | | 7 | 7000.00 | | 8 | NULL | +----+---------+ 8 rows in set (0.00 sec)
（1）当工资高于5000就显示高工资，低于5000的就显示低工资：
mysql> select id,if(salary>5000,'高工资','低工资') as level from salary; +----+-----------+ | id | level | +----+-----------+ | 1 | 低工资 | | 2 | 低工资 | | 3 | 低工资 | | 4 | 低工资 | | 5 | 低工资 | | 6 | 高工资 | | 7 | 高工资 | | 8 | 低工资 | +----+-----------+ 8 rows in set (0.00 sec)View Code
（2）当没有工资的时候，用0代替：
mysql> select ifnull(salary,0) from salary; +------------------+ | ifnull(salary,0) | +------------------+ | 1000.00 | | 2000.00 | | 3000.00 | | 4000.00 | | 5000.00 | | 6000.00 | | 7000.00 | | 0.00 | +------------------+ 8 rows in set (0.00 sec)View Code
（3）用case when来实现例子1：
mysql> select id,case when salary>5000 then '高工资' else '低工资' end from salary; +----+-------------------------------------------------------------+ | id | case when salary>5000 then '高工资' else '低工资' end | +----+-------------------------------------------------------------+ | 1 | 低工资 | | 2 | 低工资 | | 3 | 低工资 | | 4 | 低工资 | | 5 | 低工资 | | 6 | 高工资 | | 7 | 高工资 | | 8 | 低工资 | +----+-------------------------------------------------------------+ 8 rows in set (0.00 sec)
（4）当工资是1000的为特低工资，工资为2000的为低工资，其余为高工资：
mysql> select id,case salary when 1000 then '特低工资' when 2000 then '低工资' else '高工资' end from salary; +----+-------------------------------------------------------------------------------------------+ | id | case salary when 1000 then '特低工资' when 2000 then '低工资' else '高工资' end | +----+-------------------------------------------------------------------------------------------+ | 1 | 特低工资 | | 2 | 低工资 | | 3 | 高工资 | | 4 | 高工资 | | 5 | 高工资 | | 6 | 高工资 | | 7 | 高工资 | | 8 | 高工资 | +----+-------------------------------------------------------------------------------------------+ 8 rows in set (0.00 sec)
5.其他函数
MySQL内置的函数还有很多，这里就不做一一介绍，具体的可以参考官方的文档，里面也有举了很多的栗子：https://dev.mysql.com/doc/refman/5.7/en/functions.html
自定义函数
用户可以自定义函数，语法如下：
CREATE FUNCTION func_name([parameter1,parameter2...])
RETURNS type
runtime_body下面就来创建一个函数，比如自定义一个两个整数的加法函数：
DELIMITER $ CREATE FUNCTION mysum(n1 INT,n2 INT) RETURNS INT BEGIN DECLARE num int DEFAULT 0; SET num = n1 + n2; RETURN(NUM); END $ DELIMITER ;
执行函数使用SELECT：
mysql> SELECT mysum(1,2); +------------+ | mysum(1,2) | +------------+ | 3 | +------------+ 1 row in set (0.00 sec)
删除自定函数使用:
DROP FUNCTION func_name
比如删除上面创建的函数：
mysql> drop function mysum; Query OK, 0 rows affected (0.00 sec)
自定义函数暂时写到这里，更详细的会在下一篇博客中和存储过程一起介绍。
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