Assign Value to an Element in BI Publisher

Summary

Assign Value to an Element in BI Publisher

Content

I am new in using BI Publisher and I am wondering how to assign a value to an element of BI Publisher Data Model if it's null. For example in my .rtf template I have <?AMOUNT?>  which is a variable. If it's null in the database, I would like to define a default value based on If...else conditions. How can I do for example <?AMOUNT=3?>

Thanks.