Datanomics

UPDATE:

A better solution to the problem posed below is:

levelsof country, local(levels)
foreach cnt of local levels {

More information available from the Stata FAQ.

Suppose you have a database of countries over time in the long format and you want a simple list of the countries. If that dataset is open, simply run the following:

preserve
by country, sort: gen nvals = _n
keep if nvals == 1
keep country
save country_list, replace
restore