input id t1 t2 t3
1 1.5 1.4 1.6
2 1.5 . 1.9
3 . 2.0 1.6
4 . . 2.2
5 1.9 2.1 2
6 1.8 2.0 1.9
7 . . .
end


As suggested above, I want to create a new variable called sign.
If t2>t1, then sign=1; if t2<=t1, then sign=0.
Because t1 and t2 all have missing values, how can I keep missing values in the new variable "sign" in Stata?
I think the rule is as long as there is a missing value in t1 or t2, then the variable "sign" should be missing value.
Thanks for your Stata code!