Data types
The smallest unit of data that can be manipulated in SQL is called a value. How values are interpreted depends on the data type of their source.
The sources of values are as follows:
- Columns
- Constants
- Expressions
- Host variables
- Special registers
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. Although all data types include the null value, some sources of values cannot provide the null value. For example, constants, columns that are defined as NOT NULL, and special registers cannot contain null values. The COUNT function cannot return a null value column as the result of a query.