Null Semantics for Subqueries and Atomic Predicates
Narongrit Waraporn, Kriengkrai Porkaew · 2008
Abstract — SQL is one of the major languages to manipulate and retrieve data in the databases. It was standardized by the corporation among researchers and commercial database industries into many versions. SQL provides NULL values for attributes which are unknown to the user. There are three meanings of NULL but they are not classified by database engines. Results of SQL queries show different meaning when they are produced by subqueries or atomic predicates. This paper explains the meaning of them with suggestions proposing to database community.