We don’t always have perfect knowledge of the reality we are trying to model in our database. The most general approaches to handling this missing data has been either ignore it, deduce it or impute it.
Ignoring something explains itself. In SQL, we very often use a NULL when we don’t have a value. While the use of NULLs is controversial, they do give us a uniform, consistent and well-defined set of rules for handling missing values. But from a modeling perspective, there are problems with them. Aside from special computational rules and the general principle that “NULLs propagate” can