Primary key using output of function rather than column

[cjard]

I see that we can create indexes on function output instead of column value, and I wonder if we can also do that with PKs..

Basically I have a 2 part PK.. One is based on a number, one is based on whether another column is null or not (only)

So lets say for this data:

1, A
1, null
2, A
2, null

is allowed but this is NOT:

1, A
1, B


nor is this:

1, A
1, null
1, null


How do I make a PK so that it enforces the rule that:

"The combination of col1 and NVL2(col2, 0, 1) is unique"


I've tried:

ALTER TABLE myTable ADD (  CONSTRAINT myPK UNIQUE (theNumber, NVL2(theLetter, 0, 1))  ENABLE  VALIDATE  );

But it doesnt seem to work