Nulls

All data types include the null value. Distinct from all non-null values, the null value is a special value that denotes the absence of a (non-null) value.

Except for grouping operations, a null value is also distinct from another null value. Although all data types include the null value, some sources of values cannot provide the null value. For example, constants and columns that are defined as NOT NULL cannot contain null values, the COUNT and COUNT_BIG functions cannot return a null value, and ROWID columns cannot store a null value although a null value can be returned for a ROWID column as the result of a query.