MySQL之函数

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 (..)
%D             Day of the month with English suffix (0th, 1st, 2nd, 3rd, …)
%d             Day of the month, numeric (..)
%e             Day of the month, numeric (..)
%f             Microseconds (..)
%H             Hour (..)
%h             Hour (..)
%I             Hour (..)
%i             Minutes, numeric (..)
%j             Day of year (..)
%k             Hour (..)
%l             Hour (..)
%M             Month name (January..December)
%m             Month, numeric (..)
%p             AM or PM
%r             Time, -hour (hh:mm:ss followed by AM or PM)
%S             Seconds (..)
%s             Seconds (..)
%T             Time, -hour (hh:mm:ss)
%U             Week (..), where Sunday is the first day of the week; WEEK() mode
%u             Week (..), where Monday is the first day of the week; WEEK() mode
%V             Week (..), where Sunday is the first day of the week; WEEK() mode ; used with %X
%v             Week (..), where Monday is the first day of the week; WEEK() mode ; used with %x
%W             Weekday name (Sunday..Saturday)
%w             Day of the week (=Sunday..=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)

（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)

（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。