Why does Oracle filter out null if it is not equal to 1?

In Oracle, queries not equal to 1 filter out Null values ​​because: Null values ​​are not equal to any value, including 1. Oracle uses three-valued logic to handle Null values, namely true, false, and unknown. A comparison of Null to non-Null values ​​always returns Unknown unless the comparison is Null to Null (which returns True). Queries that are not equal to 1 treat Null values ​​as unknown and filter them out based on Oracle's Null handling mechanism.

Why is Null filtered out when not equal to 1 in Oracle

In Oracle, not equal to 1 ( 1) The query will filter out Null values. This is because:

• Null values ​​are not equal to any value: According to the Null truth value theorem in three-valued logic, Null The value is neither true nor false, so it is not equal to any non-Null value (including 1).
• Oracle's Null processing: Oracle uses three-valued logic (True, False, Unknown) to handle Null values. Any comparison involving a Null value returns Unknown unless the comparison is Null to Null (which returns True).

Detailed explanation:

When executing a query that is not equal to 1, Oracle will treat the Null value as unknown (Unknown). Here's how the comparison is evaluated:

• 1 1: False (because they are equal)
• 1 Null : Unknown (because Null is neither True nor False)
• Null 1: Unknown (because Null is neither True nor False)
• Null Null: True (according to the Null truth value theorem)

Since the result of Null 1 is Unknown, Oracle filters the Null value from the result dropped because the Unknown value does not meet the conditions. This is consistent with Oracle's Null handling mechanism, where Null values ​​are treated as a special value that is different from any non-Null value.

The above is the detailed content of Why does Oracle filter out null if it is not equal to 1?. For more information, please follow other related articles on the PHP Chinese website!