Make picklist unique for a value - Configuration only

Question

I have a picklist on a custom object with values A,B and C. I want only one record of the object with the value of the picklist = C. It is perfectly fine if the other records have the value A or B occurring multiple times but the value of C can only occur once. Looking to accomplish this without a trigger. Please suggest ways to accomplish this via configuration. Thanks in advance guys.

Answer 1

It would envolve a lot of configuration:

1- Create a text field and mark it as a unique field.
2- Create a workflow field update on the custom object:
    a- Entry criteria should check for the picklist value and the value should be "C"
    b- Create a field update on this workflow rule and update the unique text field to a unique value (like 'C unique').
3- Create another workflow field update on the custom object:
    a- Entry criteria should check for the picklist value and the value should NOT be "C"
    b- Create a field update on this workflow rule and update the unique text field with null.