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.

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