clear
sysuse auto

preserve
keep if foreign==1
save "foreign", replace

restore
keep if foreign==0
save "domestic", replace

clear

use "foreign"
append using "domestic"

* example without data source variable

clear
sysuse auto

preserve
keep if foreign==1
drop foreign
save "foreign", replace

restore
keep if foreign==0
drop foreign
save "domestic", replace

clear
use "foreign"
append using "domestic"

clear
use "foreign"
append using "domestic", generate(source)

* example without all the same variables

clear
sysuse auto

preserve
keep if foreign==1
drop foreign
save "foreign", replace

restore
keep if foreign==0
keep make mpg rep78
save "domestic", replace

clear
use "foreign"
append using "domestic", generate(source)

clear
use "domestic"
append using "foreign", generate(source)

* mismatched type
clear
sysuse auto

preserve
keep if foreign==1
keep make price
save "foreign", replace

restore
keep if foreign==0

generate price2 = strofreal(price)
keep make price2
rename price2 price

save "domestic", replace

clear
use "domestic"
append using "foreign", generate(source) force

clear
use "foreign"
append using "domestic", generate(source) force