These examples are about SELECT. But what about the question?
As I understand, the goal is to add such a row to table. Okay, we will decide how to UPDATE the current table.
But what about the future insertions? This field has to be calculated programmatically, or with insert/update triggers.

What is the purpose of this field? If only for unicness, then it can be done with index on two fields.

If there is no matter what type and content in this field - it is much easier just to concatenate fields with some separator. Or multiply accordigly to maximum of planned records. If there will be 1000000 records, it can be aValue*1000000+bValue.