How do I express "create new if [this data] does not already exist"?
Solution 1:
Merge is what you are looking for, at least in the SQL lexicon.
It is also called an Upsert, which indicates data can be updated where it exists and inserted where it does not.
This does not necessarily mean that existing data will be updated; existing data can be left as is.
Boring technical link
Solution 2:
perhaps you want to ensure existence of the variable?