Manipulating Nulls With SQL
As described in “Types of Missing Values” on page 617, SQL nulls are often mistakenly interpreted in ways that extend significantly beyond their single valid meaning, which is simply unknown (no value is present).
These properties make the use and interpretation of nulls in SQL problematic. The following sections outline the behavior of nulls for various SQL operations to help you to understand how to use them in data manipulation statements and to interpret the results those statements effect.
See “Manipulating Nulls With SQL” on page 626, and “NULL Literals” on page 630 for details about manipulating nulls with SQL.