*仅适用楼主例子，演练参考，解法一
inp id1 id2
1 1
3 2
5 3
8 4
end
g x=0
loc num = _N
forv i=1/`num'{
  g x`i' = (id2[`i'] == id1)
  replace x=(x`i' != x)   
  drop x`i'
}
drop if x==1

*仅适用楼主例子，演练参考，解法二，运用merge【概念与 bbwade  同】
inp id1 id2
1 1
3 2
5 3
8 4
end
preserve
g id=id1
sa "F:\id1.dta"
restore
g id=id2
drop id1
sa "F:\id2.dta"
use "F:\id1.dta"
merge 1:1 id using "F:\id2.dta"
keep if _merge==1

推荐，您是对的，我又有点错误理解楼主的意思了！

levelsof id2, local(temp)

foreach x of local temp {
drop if id1 == `x'
}

谢谢！

谢谢~！

嗯，谢谢啊！

运行中怎么有点问题：
'x' invalid name

求教！

check the left single quote, it is different from right single quote. In the most keyboard setting, the left quote is setting in the upper left corner and is next to number 1.