NULL value
260
NULL value
To specify a value that is unknown or not applicable.
NULL
Anywhere.
Must be connected to the database.
None.
"Expressions" on page 230
"Search conditions" on page 239
The NULL value is a special value which is different from any valid value
for any data type. However, the NULL value is a legal value in any data
type. The NULL value is used to represent missing or inapplicable
information. Note that these are two separate and distinct cases where NULL
is used:
Situation Description
missing The field does have a value, but that value is unknown.
inapplicable The field does not apply for this particular row.
SQL allows columns to be created with the NOT NULL restriction. This
means that those particular columns cannot contain the NULL value.
The NULL value introduces the concept of three valued logic to SQL. The
NULL value compared using any comparison operator with any value
(including the NULL value) is "UNKNOWN." The only search condition
that returns TRUE is the IS NULL predicate. In SQL, rows are selected only
if the search condition in the WHERE clause evaluates to TRUE; rows that
evaluate to UNKNOWN or FALSE are not selected.
The IS [ NOT ] truth-value clause, where truth-value is one of TRUE,
FALSE or UNKNOWN can be used to select rows where the NULL value is
involved. See "Search conditions" on page 239 for a description of this
clause.
In the following examples, the column Salary contains NULL.
Function
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Usage
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