Why Does != Fail When Comparing to NULL in SQL?

SQL's != Operator and the NULL Conundrum

SQL's != (not equal) operator presents a unique challenge when dealing with NULL values. While it effectively compares non-NULL values, its behavior with NULL is counterintuitive.

The Problem with != NULL

Consider this:

SELECT * FROM MyTable WHERE MyColumn != NULL; -- Returns 0 rows
SELECT * FROM MyTable WHERE MyColumn  NULL; -- Also returns 0 rows

These queries, aiming to find rows where MyColumn is not NULL, surprisingly return empty result sets. This isn't a bug; it's due to NULL's nature. NULL doesn't represent a value; it represents the absence of a value. Therefore, comparing a value to NULL using != or = always yields UNKNOWN, not TRUE or FALSE.

The Solution: IS NOT NULL

To accurately identify non-NULL values, use the IS NOT NULL predicate:

SELECT * FROM MyTable WHERE MyColumn IS NOT NULL; -- Returns the expected rows

IS NOT NULL (and its counterpart, IS NULL) are specifically designed for handling NULL values in SQL comparisons.

Comparing with Non-NULL Values

The != operator functions as expected when comparing with non-NULL values:

SELECT * FROM MyTable WHERE MyColumn != 'ValueX'; -- Returns rows where MyColumn is not 'ValueX'

Key Takeaway

Accurate SQL queries involving NULL values require using IS NOT NULL and IS NULL. Avoid using != or = for NULL comparisons to prevent unexpected results. Use these dedicated predicates for reliable handling of NULL in your SQL statements.

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