MySQL基础篇【单行函数】

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1. 数值函数

1.1 基本函数

举例：

SELECT ABS(-123),ABS(32),SIGN(-23),SIGN(43),PI(),CEIL(32.32),CEILING(-43.23),FLOOR(32.32),FLOOR(-43.23),MOD(12,5)
FROM DUAL;
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SELECT RAND(),RAND(),RAND(10),RAND(10),RAND(-1),RAND(-1)
FROM DUAL;
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SELECT ROUND(12.33),ROUND(12.343,2),ROUND(12.324,-1),TRUNCATE(12.66,1),TRUNCATE(12.66,-1)
FROM DUAL;
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1.2 角度与弧度互换函数

SELECT RADIANS(30),RADIANS(60),RADIANS(90),DEGREES(2*PI()),DEGREES(RADIANS(90))
FROM DUAL;
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1.3 三角函数

举例：
  ATAN2(M,N)函数返回两个参数的反正切值。 与ATAN(X)函数相比，ATAN2(M,N)需要两个参数，例如有两个点point(x1,y1)和point(x2,y2)，使用ATAN(X)函数计算反正切值为ATAN((y2-y1)/(x2-x1))，使用ATAN2(M,N)计算反正切值则为ATAN2(y2-y1,x2-x1)。由使用方式可以看出，当x2-x1等于0时，ATAN(X)函数会报错，而ATAN2(M,N)函数则仍然可以计算。

ATAN2(M,N)函数的使用示例如下：

SELECT SIN(RADIANS(30)),DEGREES(ASIN(1)),TAN(RADIANS(45)),DEGREES(ATAN(1)),DEGREES(ATAN2(1,1))
FROM DUAL;
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1.4 指数与对数

mysql> SELECT POW(2,5),POWER(2,4),EXP(2),LN(10),LOG10(10),LOG2(4)
-> FROM DUAL;
+----------+------------+------------------+-------------------+-----------+---------+
| POW(2,5) | POWER(2,4) | EXP(2)           | LN(10)            | LOG10(10) | LOG2(4) |
+----------+------------+------------------+-------------------+-----------+---------+
| 32       | 16         | 7.38905609893065 | 2.302585092994046 |     1     |    2    |
+----------+------------+------------------+-------------------+-----------+---------+
1 row in set (0.00 sec)
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1.5 进制间的转换

mysql> SELECT BIN(10),HEX(10),OCT(10),CONV(10,2,8)
-> FROM DUAL;
+---------+---------+---------+--------------+
| BIN(10) | HEX(10) | OCT(10) | CONV(10,2,8) |
+---------+---------+---------+--------------+
| 1010    |    A    |   12    |      2       |
+---------+---------+---------+--------------+
1 row in set (0.00 sec)
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2. 字符串函数

举例：

mysql> SELECT FIELD('mm','hello','msm','amma'),FIND_IN_SET('mm','hello,mm,amma') FROM DUAL;
+----------------------------------+-----------------------------------+
| FIELD('mm','hello','msm','amma') | FIND_IN_SET('mm','hello,mm,amma') |
+----------------------------------+-----------------------------------+
| 				0 				   | 				 2 				   |
+----------------------------------+-----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT NULLIF('mysql','mysql'),NULLIF('mysql', '');
+-------------------------+---------------------+
| NULLIF('mysql','mysql') | NULLIF('mysql', '') |
+-------------------------+---------------------+
| 			NULL 		  | 		mysql       |
+-------------------------+---------------------+
1 row in set (0.00 sec)
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3. 日期和时间函数

3.1 获取日期、时间

举例：

SELECT CURDATE(),CURTIME(),NOW(),SYSDATE()+0,UTC_DATE(),UTC_DATE()+0,UTC_TIME(),UTC_TIME()+0
FROM DUAL;
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3.2 日期与时间戳的转换

举例：

mysql> SELECT UNIX_TIMESTAMP(now());
+-----------------------+
| UNIX_TIMESTAMP(now()) |
+-----------------------+
|     1576380910        | 
+-----------------------+
1 row in set (0.01 sec)
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mysql> SELECT UNIX_TIMESTAMP(CURDATE());
+---------------------------+
| UNIX_TIMESTAMP(CURDATE()) |
+---------------------------+
|     1576339200            |
+---------------------------+
1 row in set (0.00 sec)
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mysql> SELECT UNIX_TIMESTAMP(CURTIME());
+---------------------------+
| UNIX_TIMESTAMP(CURTIME()) |
+---------------------------+
|       1576380969          |
+---------------------------+
1 row in set (0.00 sec)
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mysql> SELECT UNIX_TIMESTAMP('2011-11-11 11:11:11')
+---------------------------------------+
| UNIX_TIMESTAMP('2011-11-11 11:11:11') |
+---------------------------------------+
| 				1320981071              |
+---------------------------------------+
1 row in set (0.00 sec)
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mysql> SELECT FROM_UNIXTIME(1576380910);
+---------------------------+
| FROM_UNIXTIME(1576380910) |
+---------------------------+
|   2019-12-15 11:35:10     |
+---------------------------+
1 row in set (0.00 sec)
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3.3 获取月份、星期、星期数、天数等函数

举例：

SELECT YEAR(CURDATE()),MONTH(CURDATE()),DAY(CURDATE()),HOUR(CURTIME()),MINUTE(NOW()),SECOND(SYSDATE())
FROM DUAL;
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SELECT MONTHNAME('2021-10-26'),DAYNAME('2021-10-26'),WEEKDAY('2021-10-26'), 
QUARTER(CURDATE()),WEEK(CURDATE()),DAYOFYEAR(NOW()),DAYOFMONTH(NOW()),DAYOFWEEK(NOW())
FROM DUAL;
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3.4 日期的操作函数

EXTRACT(type FROM date)函数中type的取值与含义：

举例：

SELECT EXTRACT(MINUTE FROM NOW()),EXTRACT( WEEK FROM NOW()),
EXTRACT( QUARTER FROM NOW()),EXTRACT( MINUTE_SECOND FROM NOW())
FROM DUAL;
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3.5 时间和秒钟转换的函数

举例：

mysql> SELECT TIME_TO_SEC(NOW());
+--------------------+
| TIME_TO_SEC(NOW()) |
+--------------------+
| 78774              |
+--------------------+
1 row in set (0.00 sec)
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mysql> SELECT SEC_TO_TIME(78774);
+--------------------+
| SEC_TO_TIME(78774) |
+--------------------+
| 21:52:54           |
+--------------------+
1 row in set (0.12 sec)
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3.6 计算日期和时间的函数

第1组：

上述函数中type的取值：

举例：

SELECT DATE_ADD(NOW(), INTERVAL 1 DAY) AS col1,DATE_ADD('2021-10-21 23:32:12',INTERVAL 1 SECOND) AS col2,
ADDDATE('2021-10-21 23:32:12',INTERVAL 1 SECOND) AS col3,
DATE_ADD('2021-10-21 23:32:12',INTERVAL '1_1' MINUTE_SECOND) AS col4,
DATE_ADD(NOW(), INTERVAL -1 YEAR) AS col5, #可以是负数
DATE_ADD(NOW(), INTERVAL '1_1' YEAR_MONTH) AS col6 #需要单引号
FROM DUAL;
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SELECT DATE_SUB('2021-01-21',INTERVAL 31 DAY) AS col1,
SUBDATE('2021-01-21',INTERVAL 31 DAY) AS col2,
DATE_SUB('2021-01-21 02:01:01',INTERVAL '1 1' DAY_HOUR) AS col3
FROM DUAL;
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第2组：

举例：

SELECT
ADDTIME(NOW(),20),SUBTIME(NOW(),30),SUBTIME(NOW(),'1:1:3'),DATEDIFF(NOW(),'2021-10-01'),
TIMEDIFF(NOW(),'2021-10-25 22:10:10'),FROM_DAYS(366),TO_DAYS('0000-12-25'),
LAST_DAY(NOW()),MAKEDATE(YEAR(NOW()),12),MAKETIME(10,21,23),PERIOD_ADD(20200101010101,10)
FROM DUAL;
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mysql> SELECT ADDTIME(NOW(), 50);
+---------------------+
| ADDTIME(NOW(), 50) |
+---------------------+
| 2019-12-15 22:17:47 |
+---------------------+
1 row in set (0.00 sec)

mysql> SELECT ADDTIME(NOW(), '1:1:1');
+-------------------------+
| ADDTIME(NOW(), '1:1:1') |
+-------------------------+
| 2019-12-15 23:18:46     |
+-------------------------+
1 row in set (0.00 sec)
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mysql> SELECT SUBTIME(NOW(), '1:1:1');
+-------------------------+
| SUBTIME(NOW(), '1:1:1') |
+-------------------------+
| 2019-12-15 21:23:50     |
+-------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBTIME(NOW(), '-1:-1:-1');
+----------------------------+
| SUBTIME(NOW(), '-1:-1:-1') |
+----------------------------+
| 2019-12-15 22:25:11        |
+----------------------------+
1 row in set, 1 warning (0.00 sec)
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mysql> SELECT FROM_DAYS(366);
+----------------+
| FROM_DAYS(366) |
+----------------+
| 0001-01-01     |
+----------------+
1 row in set (0.00 sec)
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mysql> SELECT MAKEDATE(2020,1);
+------------------+
| MAKEDATE(2020,1) |
+------------------+
| 2020-01-01       |
+------------------+
1 row in set (0.00 sec)
mysql> SELECT MAKEDATE(2020,32);
+-------------------+
| MAKEDATE(2020,32) |
+-------------------+
| 2020-02-01        |
+-------------------+
1 row in set (0.00 sec)
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mysql> SELECT MAKETIME(1,1,1);
+-----------------+
| MAKETIME(1,1,1) |
+-----------------+
| 01:01:01        |
+-----------------+
1 row in set (0.00 sec)
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mysql> SELECT PERIOD_ADD(20200101010101,1);
+------------------------------+
| PERIOD_ADD(20200101010101,1) |
+------------------------------+
| 20200101010102               |
+------------------------------+
1 row in set (0.00 sec)
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mysql> SELECT TO_DAYS(NOW());
+----------------+
| TO_DAYS(NOW()) |
+----------------+
| 737773         |
+----------------+
1 row in set (0.00 sec)
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举例：查询 7 天内的新增用户数有多少？

SELECT COUNT(*) as num FROM new_user WHERE TO_DAYS(NOW())-TO_DAYS(regist_time)<=7
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3.7 日期的格式化与解析

上述 非GET_FORMAT 函数中fmt参数常用的格式符：

GET_FORMAT函数中date_type和format_type参数取值如下：

举例：

mysql> SELECT DATE_FORMAT(NOW(), '%H:%i:%s');
+--------------------------------+
| DATE_FORMAT(NOW(), '%H:%i:%s') |
+--------------------------------+
| 22:57:34                       |
+--------------------------------+
1 row in set (0.00 sec)
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SELECT STR_TO_DATE('09/01/2009','%m/%d/%Y')
FROM DUAL;
SELECT STR_TO_DATE('20140422154706','%Y%m%d%H%i%s')
FROM DUAL;
SELECT STR_TO_DATE('2014-04-22 15:47:06','%Y-%m-%d %H:%i:%s')
FROM DUAL;
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mysql> SELECT GET_FORMAT(DATE, 'USA');
+-------------------------+
| GET_FORMAT(DATE, 'USA') |
+-------------------------+
| %m.%d.%Y                |
+-------------------------+
1 row in set (0.00 sec)
SELECT DATE_FORMAT(NOW(),GET_FORMAT(DATE,'USA')),
FROM DUAL;
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mysql> SELECT STR_TO_DATE('2020-01-01 00:00:00','%Y-%m-%d');
+-----------------------------------------------+
| STR_TO_DATE('2020-01-01 00:00:00','%Y-%m-%d') |
+-----------------------------------------------+
| 2020-01-01                                    |
+-----------------------------------------------+
1 row in set, 1 warning (0.00 sec)

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4. 流程控制函数

流程处理函数可以根据不同的条件，执行不同的处理流程，可以在SQL语句中实现不同的条件选择。
MySQL中的流程处理函数主要包括IF()、IFNULL()和CASE()函数。

SELECT IF(1 > 0,'正确','错误')
->正确
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SELECT IFNULL(null,'Hello Word')
->Hello Word
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SELECT CASE
	WHEN 1 > 0
	THEN '1 > 0'
	WHEN 2 > 0
	THEN '2 > 0'
	ELSE '3 > 0'
	END
->1 > 0
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SELECT CASE 1
	WHEN 1 THEN '我是1'
	WHEN 2 THEN '我是2'
ELSE '你是谁'
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SELECT employee_id,salary, CASE WHEN salary>=15000 THEN '高薪'
								WHEN salary>=10000 THEN '潜力股'
								WHEN salary>=8000 THEN '屌丝'
								ELSE '草根' END "描述"
FROM employees;
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SELECT oid,`status`, CASE `status`  WHEN 1 THEN '未付款'
									WHEN 2 THEN '已付款'
									WHEN 3 THEN '已发货'
									WHEN 4 THEN '确认收货'
									ELSE '无效订单' END
FROM t_order;
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mysql> SELECT CASE WHEN 1 > 0 THEN 'yes' WHEN 1 <= 0 THEN 'no' ELSE 'unknown' END;
+---------------------------------------------------------------------+
| CASE WHEN 1 > 0 THEN 'yes' WHEN 1 <= 0 THEN 'no' ELSE 'unknown' END |
+---------------------------------------------------------------------+
| yes                                                                 |
+---------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT CASE WHEN 1 < 0 THEN 'yes' WHEN 1 = 0 THEN 'no' ELSE 'unknown' END;
+--------------------------------------------------------------------+
| CASE WHEN 1 < 0 THEN 'yes' WHEN 1 = 0 THEN 'no' ELSE 'unknown' END |
+--------------------------------------------------------------------+
| unknown                                                            |
+--------------------------------------------------------------------+
1 row in set (0.00 sec)

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mysql> SELECT CASE 1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END;
+------------------------------------------------+
| CASE 1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END |
+------------------------------------------------+
| 1                                              |
+------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT CASE -1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END;
+-------------------------------------------------+
| CASE -1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END |
+-------------------------------------------------+
| -1                                              |
+-------------------------------------------------+
1 row in set (0.00 sec)
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SELECT employee_id,12 * salary * (1 + IFNULL(commission_pct,0))
FROM employees;
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SELECT last_name, job_id, salary,
		CASE job_id WHEN 'IT_PROG' THEN 1.10*salary
					WHEN 'ST_CLERK' THEN 1.15*salary
					WHEN 'SA_REP' THEN 1.20*salary
					ELSE salary END "REVISED_SALARY"
FROM employees;
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5.加密解密函数

通常需要给字段加密时都会在存储数据库前的后台加密，并不会在MySQL中进行加密。

加密与解密函数主要用于对数据库中的数据进行加密和解密处理，以防止数据被他人窃取。这些函数在保证数据库安全时非常有用。

可以看到，ENCODE(value,password_seed)函数与DECODE(value,password_seed)函数互为反函数。
举例：

mysql> SELECT PASSWORD('mysql'), PASSWORD(NULL);
+-------------------------------------------+----------------+
| PASSWORD('mysql') | PASSWORD(NULL)                         |
+-------------------------------------------+----------------+
| *E74858DB86EBA20BC33D0AECAE8A8108C56B17FA |                |
+-------------------------------------------+----------------+
1 row in set, 1 warning (0.00 sec)
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SELECT md5('123')
->202cb962ac59075b964b07152d234b70
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SELECT SHA('Tom123')
->c7c506980abc31cc390a2438c90861d0f1216d50
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mysql> SELECT ENCODE('mysql', 'mysql');
+--------------------------+
| ENCODE('mysql', 'mysql') |
+--------------------------+
| íg ¼ ìÉ                  |
+--------------------------+
1 row in set, 1 warning (0.01 sec)
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mysql> SELECT DECODE(ENCODE('mysql','mysql'),'mysql');
+-----------------------------------------+
| DECODE(ENCODE('mysql','mysql'),'mysql') |
+-----------------------------------------+
| mysql                                   |
+-----------------------------------------+
1 row in set, 2 warnings (0.00 sec)

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6. MySQL信息函数

MySQL中内置了一些可以查询MySQL信息的函数，这些函数主要用于帮助数据库开发或运维人员更好地
对数据库进行维护工作。

举例：

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test       |
+------------+
1 row in set (0.00 sec)
mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test       |
+------------+
1 row in set (0.00 sec)
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mysql> SELECT USER(), CURRENT_USER(), SYSTEM_USER(),SESSION_USER();
+----------------+----------------+----------------+----------------+
| USER() | CURRENT_USER() | SYSTEM_USER() | SESSION_USER()          |
+----------------+----------------+----------------+----------------+
| root@localhost | root@localhost | root@localhost | root@localhost |
+----------------+----------------+----------------+----------------+
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mysql> SELECT CHARSET('ABC');
+----------------+
| CHARSET('ABC') |
+----------------+
| utf8mb4        |
+----------------+
1 row in set (0.00 sec)
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mysql> SELECT COLLATION('ABC');
+--------------------+
| COLLATION('ABC')   |
+--------------------+
| utf8mb4_general_ci |
+--------------------+
1 row in set (0.00 sec)

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7. 其他函数

MySQL中有些函数无法对其进行具体的分类，但是这些函数在MySQL的开发和运维过程中也是不容忽视
的。

举例：

# 如果n的值小于或者等于0，则只保留整数部分
mysql> SELECT FORMAT(123.123, 2), FORMAT(123.523, 0), FORMAT(123.123, -2);
+--------------------+--------------------+---------------------+
| FORMAT(123.123, 2) | FORMAT(123.523, 0) | FORMAT(123.123, -2) |
+--------------------+--------------------+---------------------+
| 123.12             | 124                | 123                 |
+--------------------+--------------------+---------------------+
1 row in set (0.00 sec)
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mysql> SELECT CONV(16, 10, 2), CONV(8888,10,16), CONV(NULL, 10, 2);
+-----------------+------------------+-------------------+
| CONV(16, 10, 2) | CONV(8888,10,16) | CONV(NULL, 10, 2) |
+-----------------+------------------+-------------------+
| 10000           | 22B8             | NULL              |
+-----------------+------------------+-------------------+
1 row in set (0.00 sec)
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mysql> SELECT INET_ATON('192.168.1.100');
+----------------------------+
| INET_ATON('192.168.1.100') |
+----------------------------+
| 3232235876                 |
+----------------------------+
1 row in set (0.00 sec)
# 以“192.168.1.100”为例，计算方式为192乘以256的3次方，加上168乘以256的2次方，加上1乘以256，再加上
100。
mysql> SELECT INET_NTOA(3232235876);
+-----------------------+
| INET_NTOA(3232235876) |
+-----------------------+
| 192.168.1.100         |
+-----------------------+
1 row in set (0.00 sec)
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mysql> SELECT BENCHMARK(1, MD5('mysql'));
+----------------------------+
| BENCHMARK(1, MD5('mysql')) |
+----------------------------+
| 0                          |
+----------------------------+
1 row in set (0.00 sec)
mysql> SELECT BENCHMARK(1000000, MD5('mysql'));
+----------------------------------+
| BENCHMARK(1000000, MD5('mysql')) |
+----------------------------------+
| 0                                | 
+----------------------------------+
1 row in set (0.20 sec)
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mysql> SELECT CHARSET('mysql'), CHARSET(CONVERT('mysql' USING 'utf8'));
+------------------+----------------------------------------+
| CHARSET('mysql') | CHARSET(CONVERT('mysql' USING 'utf8')) |
+------------------+----------------------------------------+
| utf8mb4          | utf8                                   |
+------------------+----------------------------------------+
1 row in set, 1 warning (0.00 sec)
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