Oracle® Database SQL Reference 10g Release 2 (10.2) Part Number B14200-02 |
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If a column in a row has no value, then the column is said to be null, or to contain null. Nulls can appear in columns of any datatype that are not restricted by NOT
NULL
or PRIMARY
KEY
integrity constraints. Use a null when the actual value is not known or when a value would not be meaningful.
Do not use null to represent a value of zero, because they are not equivalent.
Note:
Oracle Database currently treats a character value with a length of zero as null. However, this may not continue to be true in future releases, and Oracle recommends that you do not treat empty strings the same as nulls.Any arithmetic expression containing a null always evaluates to null. For example, null added to 10 is null. In fact, all operators (except concatenation) return null when given a null operand.
All scalar functions (except REPLACE
, NVL
, and CONCAT
) return null when given a null argument. You can use the NVL
function to return a value when a null occurs. For example, the expression NVL(commission_pct,0)
returns 0 if commission_pct
is null or the value of commission_pct
if it is not null.
Most aggregate functions ignore nulls. For example, consider a query that averages the five values 1000, null, null, null, and 2000. Such a query ignores the nulls and calculates the average to be (1000+2000)/2 = 1500.
To test for nulls, use only the comparison conditions IS
NULL
and IS
NOT
NULL
. If you use any other condition with nulls and the result depends on the value of the null, then the result is UNKNOWN
. Because null represents a lack of data, a null cannot be equal or unequal to any value or to another null. However, Oracle considers two nulls to be equal when evaluating a DECODE
function. Please refer to DECODE for syntax and additional information.
Oracle also considers two nulls to be equal if they appear in compound keys. That is, Oracle considers identical two compound keys containing nulls if all the non-null components of the keys are equal.
A condition that evaluates to UNKNOWN
acts almost like FALSE
. For example, a SELECT
statement with a condition in the WHERE
clause that evaluates to UNKNOWN
returns no rows. However, a condition evaluating to UNKNOWN
differs from FALSE
in that further operations on an UNKNOWN
condition evaluation will evaluate to UNKNOWN
. Thus, NOT
FALSE
evaluates to TRUE
, but NOT
UNKNOWN
evaluates to UNKNOWN
.
Table 2-20 shows examples of various evaluations involving nulls in conditions. If the conditions evaluating to UNKNOWN
were used in a WHERE
clause of a SELECT
statement, then no rows would be returned for that query.
Table 2-20 Conditions Containing Nulls
Condition | Value of A | Evaluation |
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For the truth tables showing the results of logical conditions containing nulls, see Table 7-5, Table 7-6, and Table 7-7.