NULL in Teradata

A NULL represents any of three things:
• An empty column
• An unknown value
• An unknowable value

Nulls are neither values nor they signify values; they represent the absence of value.
A null is a place holder indicating that no value is present.

Examples:

Casting NULL
SELECT CAST (NULL AS DATE);

NULL with CASE
SELECT CASE WHEN id= 101 THEN NULL END FROM tbl_employee;

NULL with default while creating table
CREATE TABLE tbl_employee
(
id INTEGER DEFAULT 99,
experience DEFAULT NULL
);

Note: No need to mention the datatype if you put DEFAULT NULL.

Feature of NULL:
SELECT TYPE(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 datatype of NULL.
To avoid type issues, cast NULL to the desired type.