MYSQL数据库学习十三 使用MySQL常用函数

13.1 字符串函数

对于针对字符串位置的操作，第一个位置被标记为1.

函数	功能
CONCAT(str1,str2...strn)	连接字符串str1、str2、...strn
INSERT(str,pos,len,newstr)	将字符串str从第pos位置开始，长度为len的字符串替换为字符串newstr
LOWER(str)/LCASE(str)	将字符串str中所有字符变为小写
UPPER(str)/UCASE(str)	将字符串str中所有字符变为大写
LEFT(str,x)	返回字符串str最左边的x个字符
RIGHT(str,x)	返回字符串str最右边的x个字符
LPAD(str,n,pad)	使用字符串pad对字符串str最左边进行填充，直到长度为n个字符长度
RPAD(str,n,pad)	使用字符串pad对字符串str最右边进行填充，直到长度为n个字符长度
LTRIM(str)	去掉字符串str开始处的空格
RTRIM(str)	去掉字符串str借书处的空格
TRIM(str)	去掉字符串str开始和结束处的空格
REPEAT(str,x)	返回字符串str重复x次的结果
REVERSE(str)	返回颠倒字符串str顺序的结果
REPLACE(str,substr,newstr)	使用字符串b替换字符串str中所有出现的字符串a
STRCMP(str1,str2)	比较字符串str和字符串str2
SUBSTRING(str,num,len)	返回字符串str从num位置起len个字符长度的字符串
MID(str,num,len)
LENGTH(str)	获取传入的参数str的长度
CHAR_LENGTH(str)	获取传入的参数str的字符数
FIND_IN_SET(str1,str2)	返回在str2中与str1相匹配的字符串的位置，str2中包含若干个用逗号分隔的字符串
FIELD(str,str1,str2...)	返回str1、str2、...中第一个与字符串str匹配的字符串的位置
LOCATE(str1,str)	返回参数str中字符串str1开始的位置
POSITION(str1 IN str)
INSERT(str,str1)
ELT(n,str1,str2...)	返回str1,str2...中第n个字符串
MAKE_SET(num,str1,str2...strn)	将num转换成二进制，从右到左读取值来选择字符串，值为1选择，值为0不选择

CONCAT(S1，S2，...SN) //如果其中一个参数为NULL，则返回值为NULL
CONCAT_WS(SEP,S1，S2，...SN)  //分隔符SEP会将各个字符串分隔开。当SEP的值为NULL时，返回NULL；当分隔符参数后存在NULL时，返回的结果将忽略NULL。
STRCMP(str1,str2)  //当str1大于str2，返回1；str1等于str2，返回0；str1小于str2，返回-1.

13.2 数学函数

函数	功能
ABS(x)	返回x的绝对值
FLOOR(x)	返回不大于x的最大整数值
CEIL(x)	返回不小于x的最小整数值
EXP(x)	返回e的x次方
SQRT(x)	返回非负数x的二次方根
LN(x)	返回x的自然对数
MOD(x,y)	返回x模y的值
RAND()/RAND(N)	返回0~1之间的随机数，N为种子
ROUND(x)/ROUND(x,d)	返回数值x四舍五入后的整数/d位小数
TRUNCATE(x,d)	返回数值x截断为d位小数，d可以为负数
FORMAT(x,d)	返回数值x四舍五入后有d位小数的字符串

13.3 日期和时间函数

	函数	功能
	CURDATE()	获取当前日期 格式：2015-07-07
	CURRENT_DATE()
	CURTIME()	获取当前时间 格式：15:51:07
	CURRENT_TIME()
	NOW()	获取当前的时间和日期 格式:2015-07-07 15:51:07
	CURRENT_TIMESTAME()
	LOCALTIME()
	SYSDATE()
	UNIX_TIMESTAMP(date)	获取日期DATE的UNIX时间戳
	FROM_UNIXTIME()	获取UNIX时间戳的日期值
	UTC_DATE()	获取UTC日期
	UTC_TIME()	获取UTC时间
	EXTRACT(type from NOW())	获取日期和时间的各部分值
	TO_DAYS(date)	计算date与0000年1月1日相隔的天数
	FROM_DAYS(number)	0000年1月1日经历number天后的日期
	DATEDIFF(date1,date2)	date1和date2相隔的天数
	ADDDATE(date,n)	date加上n天后的日期
	SUBDATE(date,n)	date减去n天后的日期
*	ADDDATE(d,INTERNAL expr type)	d加上一段时间后的日期，expr决定了时间的长度，type决定了操作的对象
	ADDDATE(d,INTERNAL expr type)	d减去一段时间后的日期，expr决定了时间的长度，type决定了操作的对象
	ADDTIME(time,n)	time加上n秒后的时间
	SUBTIME(time,n)	time减去n秒后的时间
	WEEK()	返回当前年的第几周
	WEEKOFYEAR()
	DAYOFWEEK()	返回星期几，1~7，1表示星期日
	WEEKDAY()	返回星期几，0~6，0表示星期一
	DAYNAME()	返回英文表示的星期
	YEAR(date)	返回日期date的年份
	HOUR(time)	返回时间time的小时值
	MINUTE(time)	返回时间time的分钟值
	SECOND(time)	返回时间time的秒
	MONTH()	返回月份1~12
	MONTHNAME()	返回月份的英文
	DAYOFYEAR()	年中第几天
	DAYOFMONTH()	月中第几天

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UTC（国际协调时间）返回的时间与当前时间相差几个时区。

*参数type的值

type的值	expr的表达式
YEAR	YY
MONTH	MM
DAY	DD
HOUR	hh
MINUTE	mm
SECOND	ss
YEAR_MONTH	YY与MM之间用任意符号隔开
DAY_HOUR	DD与hh之间用任意符号隔开
DAY_MINUTE	DD与mm之间用任意符号隔开
DAY_SECOND	DD与ss之间用任意符号隔开
HOUR_MINUTE	hh与mm之间用任意符号隔开
HOUR_SECOND	hh与ss之间用任意符号隔开
MINUTE_SECOND	mm与ss之间用任意符号隔开

13.4 系统信息函数

函数	功能
VERSION()	返回数据库的版本号
DATABASE()	返回当前数据库名
USER()	返回当前用户名
LAST_INSERT_ID	返回最近生成的AUTO_INCREMENT的ID值