Using a stage is a nice way to manipulate source sets with operatos like UNION, MINUS, INTERSECT, etc.
- Step 1 : add the desired source table to the mapping
- Step 2 : add a stage
- Step 3 : on the stage, add Sets and map source columns to each set
- Step 4 : in the expression editor of the stage, specify the desired expression, for example: [A] union ( [B] minus [C] )