Hi, professor I have a question about the standard of combination. You mention that : "HINT: for each of the 3 input files, create a temporary version with an extra column that combines country code and year. You can then use join to join two of the 3 files, and then the third with the new combination." I notice that there are different ways, such as inner, Left, right and outer join, which will influence the output. I have no idea which one should I choose, as the sample2.tsv just has few data. In the output file, should it include all the "code + year" from three files, even if some files don't have this match key's data? or just base on one file to match, making sure this code year has data? thank you.

Dear Professor， Can a string column contain numbers? Thanks for for your clarification!