c＃从数据库中过滤掉NULL行并更新existig行

In a table of my database I want to create new entries that don't exist and update them if they do exist. I got a table that links the UserID LernfeldID and Schoolyear to the Marks. There should only be a database insert when there is no match for the UserID which means that the entry does not exist. If there is an entry it should get updated. In a table of my database I want to create new