As described in Types of Missing Values, 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 SQL operations to help you to understand how to use nulls in data manipulation statements and to interpret the results those statements effect.
See NULL Literals for more details about manipulating nulls with SQL.