优雅的R-利用sqldf对dataframe进行sql操作，利用sqldf实现数据分组求平均，惊艳

> s<-paste('avg(v',seq(from=2,to=length(test)-1),")",sep = "")
> s1<-paste(s,sep="",collapse = ",")
> s2<-paste("select v1,",s1,"from test group by v1",sep = " ")
> sqldf(s2)->newdf
> View(newdf)