MYSQL函数记录20210902（截取字段计算总和求平均值）

表数据

select * from test;

1.截取

1.1 substring_index（关键字的索引开始截取，-1由最后往前截）

1.1.1 从"("开始截取

select substring_index(memory_usage,'(',-1) as substr from test;

1.1.2 再次截取（1 从最前完后截取，截取到%之前）

select substring_index(substring_index(memory_usage,'(',-1),'%',1) as substr from test;

1.2 substring

配合获取索引的函数 instr()以及left 从左开始截取到某个索引为止， 同样可以实现

1.2.1 INSTR

select substring(memory_usage,INSTR(memory_usage,'(')+1) from test;

1.2.2 left

select left(substring(memory_usage,INSTR(memory_usage,'(')+1),INSTR(substring(memory_usage,INSTR(memory_usage,'(')+1),'%')-1) from test;

2.累加计算求平均

select sum(substring_index(substring_index(memory_usage,'(',-1),'%',1))/(select count(1) from test) as avgstr from test;

3.取两位小数

select round(sum(substring_index(substring_index(memory_usage,'(',-1),'%',1))/(select count(1) from test),2) as avgstr from test;

4.拼接%

select concat(round(sum(substring_index(substring_index(memory_usage,'(',-1),'%',1))/(select count(1) from test),2),'%') avgstr from test;

5.非0判断，分母不能为0，null->0 （select count(1) from test）

select 
	case when  (select count(1) from test)>0 
	then  concat(round(sum(substring_index(substring_index(memory_usage,'(',-1),'%',1))/(select count(1) from 			test),2),'%')
	else 0 end  as avgstr
from test;

其它：
CAST函数用于将某种数据类型的表达式显式转换为另一种数据类型。
语法：CAST (expression AS data_type)