I’m afraid that *”that is in which restaurant we met yesterday" is not proper English, and would confuse native speakers. Note that your original sentence is in formal style. Here are some more typical ways to word it in today's English: A. ”That is the restaurant (that) we met in yesterday." B. ”That is the restaurant which we met in yesterday." C. ”That is the restaurant where we met yesterday." My favorite option is A, which makes it very clear the relative clause is restrictive / defining. I also prefer to omit the optional “that” — never use extra words unless you have a good reason. “That is the restaurant we met in yesterday.”

Hi Ivan, Your sentence: "that is in which restaurant we met yesterday?" No, the sentence structure is incorrect. It seems to me that you are trying to transform your original sentence (That is the restaurant in which we met yesterday.) into a question. We often start a question in English using auxiliary verbs such as "Is". We also use "where" as a relative pronoun to provide more information about a location. "Is (auxiliary verb to start a question) that (imagine that you are pointing to the restaurant) the restaurant (preceding noun) where we met yesterday?" Just as a side discussion, you can also write your original sentence as: "That is the restaurant [where] we met yesterday." I hope this helps you.

That is perfect formal English. We don't speak like that on a daily basis. We would say 'that is the restaurant we met at/in yesterday' or 'that is the restaurant we met at/in yesterday' but then again it depends on the situation and who we are speaking with. Oops! I didn't read the rewritten sentence. So, the answer is no.