subset() 函数：
subset(dataset , subset , select )
dataset 是 要进行操作的数据集
subset 是对数据的某些字段进行操作
select 选取要显示的字段
Example :
```{r spdep}
hightech
aa = subset(hightech,year==2001&province=="北京",select = c("year","province","PAT"))
bb = subset(hightech,year==2001|province=="北京",select = c("year","province","NPO"))
aa
bb
```
aa:
bb:
