Quite often a do-file is written to run on various data-files that all seem to be the same. Say, you have done it for one year of a data-set and want to repeat the same for the subsequent years that you have data. Now, a check whether the data actually has some data (given your selection) is often a good idea, here is how I did it in a recent project:
count
assert `r(N)'>1
I am using the saved return local variable r(N) that STATA automatically generates after count. This is a need feature that you should consider for many other commands (try return list after your favourite command).