The principle that Nulls aren't equal to each other (but rather that the result is Unknown) is effectively violated in the SQL specification for the codice_58 operator, which does identify nulls with each other. Consequently, some set operations in SQL, like union or difference, may produce results not representing sure information, unlike operations involving explicit comparisons with NULL (e.g. those in a codice_3 clause discussed above). In Codd's 1979 proposal (which was basically adopted by SQL92) this semantic inconsistency is rationalized by arguing that removal of duplicates in set operations happens "at a lower level of detail than equality testing in the evaluation of retrieval operations."