- An empty column
- An unknown value
- An unknowable value
Nulls are neither values nor do they signify values; they represent the absence of value. A null is a place holder indicating that no value is present.
NULL Keyword
The keyword NULL represents null, and is sometimes available as a special construct similar to, but not identical with, a literal.
ANSI Compliance
NULL is ANSI/ISO SQL:2011-compliant with extensions.
Using NULL as a Literal
- A CAST source operand, for example:
SELECT CAST (NULL AS DATE);
- A CASE result, for example.
SELECT CASE WHEN orders = 10 THEN NULL END FROM sales_tbl;
- An insert item specifying a null is to be placed in a column position on INSERT.
- An update item specifying a null is to be placed in a column position on UPDATE.
- A default column definition specification, for example:
CREATE TABLE European_Sales (Region INTEGER DEFAULT 99 ,Sales Euro_Type DEFAULT NULL);
- An explicit SELECT item, for example:
SELECT NULL
This is a Teradata extension to ANSI.
- An operand of a function, for example:
SELECT TYPE(NULL)
This is a Teradata extension to ANSI.
Data Type of NULL
When you use NULL as an explicit SELECT item or as the operand of a function, its data type is INTEGER. In all other cases NULL has no data type because it has no value.
For example, if you perform SELECT TYPE(NULL), then INTEGER is returned as the data type of NULL.
To avoid type issues, cast NULL to the desired type.
Related Topics
For information on the behavior of nulls and how to use them in data manipulation statements, see “Manipulating Nulls.”