OR(NOT(IN([_THIS], Table[ColumnName])), [_THIS] = [_THISROW_BEFORE].[_THIS])
…with the second part being required to allow updates to other values in the record using a form, without invalidating the value we are keeping unique (and not also changing) which would block the user from saving the form.
this does NOT seem to allow any value to be entered in the constrained column by the Data: set the values of some columns in this row action.
Does anyone know why?
NOT(IN([_THIS], Select(Table[ColumnName], [Key] <> [_THISROW])))
…which I think is nicer anyway.
I also noticed that neither of these two expressions seem to allow the value to be set using the new INPUT() function.
Would love to get other peoples ideas on these two methods, and if there’s other methods people use?
(I fully expect @Steve to come up with something twice as elegant, maybe even half as long!)