Multi-select string operation
I have three columns which have the same drop down options: requested, received and pending. I would like the "pending" column to have all options from the "requested" column except for what's in the "received" column.
For example: under "requested": option 1,2,3,4; under "received": option 1,2; under "pending" automaticlly pop up option 3,4.
Basiclly I want to do a minus operation between "requested" and "received" and the results are also in multi-select form.