subset函数，从某一个数据框中选择出符合某条件的数据或是相关的列

1）单条件查询

df<-data.frame(Age=c(22,24,25,26),Gender=c("Girl","Girl","Boy","Boy"),City=c("Shanghai","Beijing","Guangzhou","Nanjing"))

result<-subset(df,Gender=="Girl")

2）指定显示列

reuslt<-subset(df,Gender=="Girl",select=c(City))

3）多条件查询

result=subset(df,Age>22 & Gender=="Girl",select=c(Age,Gender))

result