How would I write..... If AdditionalPhoto:Note is null then use Product:StyleName otherwise use AdditionalPhoto:Note
The context in which you want to do this is missing. This could be a calculation in a calculaiton field, a text field with an auto-enter calculation or a script step.
The calculation field version might be:
If ( IsEmpty ( AdditionalPhoto::Note ) ; Product::StyleName ; AdditionalPhoto::Note )
But note that you have fields from two table occurrences. THe above calculation will only work if there are valid relationships linking the two occurrences.
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