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NULL and UNKNOWN (Transact-SQL)

  • Článek

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW) SQL analytics endpoint in Microsoft Fabric Warehouse in Microsoft Fabric

NULL indicates that the value is unknown. A null value is different from an empty or zero value. No two null values are equal. Comparisons between two null values, or between a null value and any other value, return unknown because the value of each NULL is unknown.

Null values generally indicate data that is unknown, not applicable, or to be added later. For example, a customer's middle initial might not be known at the time the customer places an order.

Consider:

  • To test for null values in a query, use IS NULL or IS NOT NULL in the WHERE clause.

  • You can insert null values into a column by explicitly stating NULL in an INSERT or UPDATE statement, or by leaving a column out of an INSERT statement.

  • Null values can't be used as information that is required to distinguish one row in a table from another row in a table. Examples include primary keys, or for information used to distribute rows, such as distribution keys.

Remarks

When null values are present in data, logical and comparison operators can potentially return a third result of UNKNOWN instead of just TRUE or FALSE. This need for three-valued logic is a source of many application errors. Logical operators in a boolean expression that includes UNKNOWN return UNKNOWN, unless the result of the operator doesn't depend on the UNKNOWN expression. These tables provide examples of this behavior.

The following table shows the results of applying an AND operator to two Boolean expressions where one expression returns UNKNOWN.

Expression 1 Expression 2 Result
TRUE UNKNOWN UNKNOWN
UNKNOWN UNKNOWN UNKNOWN
FALSE UNKNOWN FALSE

The following table shows the results of applying an OR operator to two Boolean expressions where one expression returns UNKNOWN.

Expression 1 Expression 2 Result
TRUE UNKNOWN TRUE
UNKNOWN UNKNOWN UNKNOWN
FALSE UNKNOWN UNKNOWN

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