Busy with training module:
Data Engineer Basics - Transform and Contextualize > CDF Transformations > Assets transformation
I have the following SQL:

The Preview works:

But, doing Run, leads to an error.

So, of course the NULL in the 1st row above ought to be “ “ for the root asset.
Q1. How do I test for NULL and set NULL value to “ ” in the Spark SQL query?
Thank you!
Regards,
Doug