How to handle a missing file in a U-SQL EXTRACT statement?

Question

I have the following EXTRACT statement that grabs the data from both a staging file and the production file:

DECLARE @staging  string = "/staging/events.csv";
DECLARE @production string = "/production/events.csv";

@events =
        EXTRACT dimensionId         string
              , tenantId            string 
              , internalEntityId    long
              , name                string
              , modifiedTimestamp   DateTime
        FROM @staging, @production
        USING Extractors.Csv();

The statement will fail if either one of the two files is missing, which causes the whole Azure Data Factory pipeline to fail:

Question

How can I gracefully handle a missing file in a U-SQL EXTRACT statement?

Answer 1

I am not sure I can qualify this as "Gracefully" but at least it does the trick.

I can use the FILE.EXISTS statement to test the presence of the file beforehand and choose to include it or not:

DECLARE @staging  string = "/staging/events.csv";
DECLARE @production string = "/production/events.csv";

IF FILE.EXISTS(@production) == true THEN
 @events =
     EXTRACT    dimensionId         string
              , tenantId            string 
              , internalEntityId    long
              , name                string
              , modifiedTimestamp   DateTime
        FROM @staging, @production
        USING Extractors.Csv();
ELSE
@events =
     EXTRACT    dimensionId         string
              , tenantId            string 
              , internalEntityId    long
              , name                string
              , modifiedTimestamp   DateTime
        FROM @staging
        USING Extractors.Csv();
END;