Yes, perhaps "key" is the wrong term. I intended to refer to the column that is used in the join condition in not so many words. > It's almost always incorrect

Yes, perhaps "key" is the wrong term. I intended to refer to the column that is used in the join condition in not so many words. > It's almost always incorrect not to have at least one JOIN condition involve a key. Eh, almost always seems a little strong. Sometimes you can reduce the number of joins you need to do by "skipping" a table that would serve as the unique key on both sides of the join. ie. instead of joining: `A --[many-to-one]--> B <--[one-to-many]-- C` You can directly join on `A.b_id = C.b_id`, and that's perfectly valid.