SQL Server触发器 - 一旦另一个字段不为空，则更新一个字段

I'm fairly new to triggers and I'm trying to update Field1 if it's null with the value from Field2 as long as Field2 isn't null on an update or insert. In other words, I want to retain the first non null value from "NonAccrual" and store it in "OriginalNonAccrual". Once OriginalNonAccrual gets a value, it should remain the same indefinitely even if NonAccrual changes. I don't know if one trigger can handle both an insert and an update and hopefully the code below gives you the idea of what I'm I'm fairly new to triggers and I'm trying to up