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.
If 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
NULL values are treated differently from other values.
NULL is used as a placeholder for unknown or inapplicable values.
Note: It is not possible to compare NULL and 0; they are not equivalent.
Look at the following "Persons" table:
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.
How do we select only the records with NULL values in the "Address" column?
We will have to use the IS NULL operator:
The result-set will look like this:
Hint: Always use IS NULL to look for NULL values.
How do we select only the records with no NULL values in the "Address" column?
We will have to use the IS NOT NULL operator:
In the next chapter we will look at the ISNULL(), NVL(), IFNULL() and COALESCE() functions.
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