mysql小白之旅——基础篇4——常用函数

参考：https://www.techonthenet.com/mysql/functions/index.php

1.非统计函数

	函数
字符串函数	concat(s1,s2...)	拼接字符串
	insert(str,x,y,instr)	指定位置插入字符串
	lower(str)
	upper(str)
	left(str,x)	取得左边的部分
	right(str,x)
	lpad(str,n,pad)
	rpad(str,n,pad)
	ltrim(str)
	rtrim(str)
	repeat(str,x)
	replace(str,a,b)
	strcmp(s1,s2)
	trim(str)
	substring(str,x,y)
	ASCII('t')	字母转换为ASCII(编码
数值函数	abs(x)
	cell(x)
	floor(x)
	mod(x,y)
	rand(x)
	round(x)
	DEGREES( number )	弧度转换为度数
	RADIANS( number )	度数转换为弧度
	SIN|COS|TAN|COT|..
	LOG|LOG2|LOG10	LOG( number ) 自然对数 或者LOG( base, number )
	POW|SQRT|EXP|LN
日期和时间函数	select curdate()	2018-09-20
	curtime()	21:59:20
	now()	2018-09-20 21:59:48
	unix_timestamp()	1537451979
	from_unixtime(1537451979)	2018-09-20 21:59:39
	week(now())	37
	year(now())	2018
	hour(now())	22
	minute(now())	1
	monthname(date)
	date_format(date,fmt)	https://www.techonthenet.com/mysql/functions/date_format.php
	date_add(date,interval expr type) date_sub adddate subdate add_time sub_time period_add	https://www.techonthenet.com/mysql/functions/date_add.php
	datediff(expr,expr2) timediff period_diff	https://www.techonthenet.com/mysql/functions/datediff.php
流程函数	if(value,t,f)
	ifnull(value1,value2)
	case when[value1]then[result1]...else[default]END
其他常用函数	DATABASE()
	VERSION()
	USER()
	INET_ATON(ip)
	INET_NTOA(num)
	PASSWOD(str)
	MD5()
	CRC32
	SHA1
高级函数	SELECT CONV(5, 10, 2);	进制转换
	LAST_INSERT_ID( [expression] )	https://www.techonthenet.com/mysql/functions/last_insert_id.php
	NULLIF	https://www.techonthenet.com/mysql/functions/nullif.php

2.统计函数

COUNT、SUM、MIM、MAX、AVG

SELECT COUNT(aggregate_expression)
FROM tables
[WHERE conditions];

--  复杂

SELECT expression1, expression2, ... expression_n,
       COUNT(aggregate_expression)
FROM tables
[WHERE conditions]
GROUP BY expression1, expression2, ... expression_n;