Do you have access to the Product Documentation. Specifically the “About Object Connection Transformation “ page?

Hello Mayura,

Yes, I didn’t notice that there’s explanation. 
I think I got the answer for my question. Thanks!

“For some scenarios it is not possible to map the target and source columns due to the fact that the source columns are not included in the target database view. You then have two options which both require some programming work. You can either extend the target view with the necessary columns or create a stored function that makes the actual key transformation. The name of the function should in this case be registered as the transformation method. Note that it should also hold the package name, e.g. My_Package_API.My_Transformation. It needs to be defined as a function that returns a string value. It also requires two values in parameters (strings); the Target Key Reference and the Service Name (in that specific order).”