In case (that) John comes/should come,please tell him to wait.中前一句话是什么从句?

问题描述:

In case (that) John comes/should come,please tell him to wait.中前一句话是什么从句?
that为什么可加可不加?

条件状语从句
in case that..万一、假使的意思
句意:如果John来的话,请告诉他等一下.
因为在从句中,that其实不作任何句子成分,跟宾语从句一样可以省略