不一定啊.
如:What you like reading is my favorite.你喜欢读的就是我所喜欢的．这是一个由 What引导的主语从句．
She wears the pretty skirt (that) she bought yesterday.她穿着昨天买的那条裙子．这是一个由that引导的宾语从句,因为是宾语从句,that可要可不要．