Limitations on NULL Values

Go Up to Selecting Rows with NULL Values

Because InterBase treats NULL values as non-values, the following limitations on NULL values in queries should be noted:

• Rows with NULL values are sorted after all other rows.
• NULL values are skipped by all aggregate operations, except for COUNT(*).
• NULL values cannot be elicited by a negated test in a search condition.
• NULL values cannot satisfy a join condition.

NULL values can be tested in comparisons. If a value on either side of a comparison operator is NULL, the result of the comparison is Unknown.

For the Boolean operators (NOT, AND, and OR), the following considerations are made:

• NULL values with NOT always returns Unknown.
• NULL values with AND return Unknown unless one operand for AND is false. In this latter case, False is returned.
• NULL values with OR return Unknown unless one operand for OR is true. In this latter case, True is returned.

For information about defining alternate NULL values, see the Data Definition Guide.