I do not believe you can do that in a formula, but I think you would have to do in in a SQL Expression Field or a Command. But the MS-SQL syntax would look something like this (I am assuming that field 1 is from another table).
SELECT CASE WHEN field1 IS NULL THEN (SELECT TOP 1 aaa FROM myTable ELSE
(SELECT TOP 1 bbb from myTable2) END
FROM ANOTHERTABLE
FROM