What is a NULL entity in SQL?

Null is an entity in SQL that is used to specify that a value does not exist. It is a reserved keyword in SQL.

Most SQL languages use the expression IS NULL in the where clause for matching records with NULL values, instead of comparing with NULL using an equality operator. See code example below:

SELECT FIRSTNAME FROM T_STUDENTS WHERE LASTNAME IS NULL;

Following functions handle Null values:

Coalesce - This function returns the first non NULL value from a list of values.
NullIf - This function accepts 2 paramaters. If both parameters are equal, NULL is returned, else the first paramater's value is returned.