"Which table is it?"
Why doesn't the sentence need は or が? I thought when you have a noun into the sentence you would always need some of those particles, or any particle at all indentifying that noun (In this case, I'm not counting the か at the end, because it indentifies it as a question, not the noun)
You can't place a particle directly before the copula です
です is used to make A=B sentence structures. AはBです
Here the topic, A, is implied (the "it" that 'which table is'), and the table is B, the description of A directly connected to です
「Aは」どのテーブルです - [it] "is which table"