Why Does != Fail to Compare Against NULL in SQL, and What Should I Use Instead?

SQL's != Operator and NULL: A Common Misunderstanding

In SQL, using != (or <>) to check for NULL values leads to unexpected results. Unlike IS NOT NULL, which correctly identifies non-NULL values, != and <> return no matches when compared to NULL. This behavior stems from the fundamental nature of NULL in SQL.

NULL represents the absence of a value, not a specific value itself. Therefore, standard comparison operators like != and <> cannot determine equality or inequality with NULL because there's no defined value to compare against.

SQL employs the IS NULL and IS NOT NULL predicates to handle comparisons with NULL. These predicates assess the existence or non-existence of a value, not its equality to another value.

This is consistent across major SQL databases, not just SQL Server. It's a direct consequence of NULL's definition as a value's absence.

Key Points to Remember:

• Use IS NOT NULL to check if a value is not NULL.
• Use = 'YOUR_VALUE' to compare a value against a specific, non-NULL value.
• Direct equality or inequality checks (=, !=, <>) with NULL are undefined and will not yield the expected results. Use IS NULL or IS NOT NULL instead.

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