If you choose the Retain type in a transform process, only the selected columns will be present in the result. This is important for Reduce Transform, which requires corresponding rows to have the same values in order to be grouped together. Use Retain Transform in conjunction with Reduce Transform to keep only the relevant columns and make sure the grouping can function.
The following table shows an example of the input:
|Field 1||Field 2||Field 3||Field 4||Field 5|
The following table shows an example of the output (Selected columns: Field 1 and Field 5):
|Field 1||Field 5|