mysql实用的函数

1. round(x,d)

把x保留d位小数

2. period_diff()

相隔月/年
description

period_diff(date_format(current_date,"%Y%m"),date_format("2016-7-6","%Y%m"))

note
不能是相隔的天数,不然计算出来的数有问题

period_diff(date_format(current_date,"%Y%m%d"),date_format("2010-09-01 00:00:00","%Y%m%d")) 

datediff()
计算两个日期之间的天数
datediff(startDate,endDate)

datediff(date_format(current_date,"%Y%m%d"),date_format("2010-09-01","%Y%m%d"))

3. 截取字段

从左开始截取字符串， left(str,length)

select left("abcdefg",3) //"abc"

从右开始截取字符串，right(str,length)

select right("abcdef",3);

截取字符串， substring(str,startPoint,length)

substring("abcdef",2,4) //bcde

4. date_sub()N天前

语法

date_sub(date,interval expr type)

示例

select date_sub(current_date,interval 180 day);

5. 最近6个月

select time between date_sub(current_date,interval 180 day) and current_date 

升序esc，降序desc