* subset columns
sysuse nlsw88

drop age

keep wage union

* subset rows
clear
sysuse nlsw88

summarize wage if union

keep if union
* save
summarize wage
tab married

* use

clear
sysuse nlsw88

preserve // make it temporary
keep if union
* save
summarize wage
tab married
restore
summarize wage

* random observations
sysuse nlsw88, clear

generate x = runiform()
sort x

keep in 1/100 // random N


sysuse nlsw88, clear

generate x = runiform()
keep if x < .1 // random proportion

sysuse nlsw88, clear
keep if runiform() < .1 // random proportion