SQL NULL Values

SQL NULL Values

NULL values represent missing unknown data.
By default, a table column can hold NULL values.
This chapter will explain the IS NULL and IS NOT NULL operators.

SQL NULL ValuesIf a column in a table is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value.

NULL values are treated differently from other values.
NULL is used as a placeholder for unknown or inapplicable values.


SQL Working with NULL Values
Look at the following "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola		Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari		Stavanger

Suppose that the "Address" column in the "Persons" table is optional. This means that if we insert a record with no value for the "Address" column, the "Address" column will be saved with a NULL value.

How can we test for NULL values?

It is not possible to test for NULL values with comparison operators, such as =, <, or <>.

We will have to use the IS NULL and IS NOT NULL operators instead.

SQL IS NULLHow do we select only the records with NULL values in the "Address" column?
We will have to use the IS NULL operator:

SELECT LastName,FirstName,Address  FROM Persons      WHERE Address IS NULL

The result-set will look like this:

LastName	FirstName	Address
Hansen	Ola
Pettersen	Kari

Note: It is not possible to compare NULL and 0; they are not equivalent.