合并两个数据。表，但不更新特定的列

I have a data.table question where I am not sure if this requires two separate operations or not. I have two data.tables I want to merge by specific columns. However, if there is a match between the keys, I only want to update some of the columns in the result. If there is no match then I want to append the new data to the result. The column names will always be the same, so there is no filling necessary.I have a data.table question where I am not sur