This question already has an answer here:
- Benutzt man Artikel mit Toponymen? 5 answers
Why is it that there is no article here:
Mein Freund hat um 15 Uhr einem Termin IN Hamburg
While we see one here:
Wo Arbeitet dein Freund ? IN DER Schweiz
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Geographical entities that have a female or male gender are used with the article. The rest usually goes with "nach" and without the neuter "das", nowever, as CarstenSchultz mentioned in a comment, there are exception for which you do have to use the article. Whether or not that is specific to places in Germany, I don't know.
You should change the question to point out you are confused be the "DER" in "in der Schweiz" This is a problem i never thought of.
You would say "In Holland", too.. but "In Schweiz" is wrong. Locations that are not neutral (gender) require an article (as Emanuel said)
Further Examples are: In den Niederlanden. In den USA. (while these are plurals, i don't know any other examples like Schweiz)
And be careful with islands, as island do not use "in" but "auf" - "Auf den Seychellen", "Auf Madagaskar" etc.