定语从句先行词​"要用which别着急,介词前面逗隔离”

这句口诀不是说先行词,是说用which引导的定语从句,其中有一种引导方式是：介词+which,这种情况如果是非限制性定语从句,前边加“逗号”---即：,介词+which
Last week,I went to New York,in which my father works.
如果是限制性定语从句,不用逗号隔开.此口诀有缺陷.
Gun control is a subject about which Americans have argued for a long time.
再问： 那就是限定性定语从句和非限定性定语从句区别？跟which有关吗？
再答： 逗号有无是限定性与非限定性造成的，与which无关。