These schemes use standardized Default Values rather than Nulls to represent missing information. They also propose elimination of 3 Valued Logic in detection of unknown answers.
Unknown answers occur when data of interest is missing.
No serious Default Value Schemes have appeared in products, but these two are most important proposals. Even so, they are both incomplete and cannot be implemented as defined. The two proposals are reviewed briefly below. In Date's scheme, each column domain can have a Default Value defined. The Default Value is a special value, not part of the actual logical domain of the column, but part of the physical domain.