A couple things:

This is admittedly a bit pedantic, but in E.F. Codd's original paper, he defined "relation" as the relation between a tuple (table row) and an attribute (table column) in a single table - https://en.wikipedia.org/wiki/Relation_(database). I'm not sure of the author's intent, but the user table example (user, country_id ) might imply the relationship between the user table and the country table. It's a common misconception about what "relational" means, but tbh I'm fine with that since it makes more sense to the average developer.

If you ever need to join sets of data in code, don't use nested loops - O(n^2). Use a map / dictionary. It's one of the few things I picked up doing Leetcode problems that I've actually needed to apply irl.