How Does MySQL Handle Inequality Comparisons with NULL Values?

Comparison of MySQL and NULL values

When dealing with table columns that may contain NULL values, it is important to understand how MySQL handles comparisons involving NULLs.

The problem occurs when using inequality comparison (CODE!='C'), but the NULL value is not considered. In this case, MySQL will silently exclude records containing NULL values ​​from the result set, making it appear that these records do not exist.

In order to include records containing NULL values ​​in the result set, the comparison must be explicitly extended to account for NULL. The correct way is to use the following syntax:

SELECT * FROM TABLE WHERE CODE IS NULL OR CODE!='C'

By using the IS NULL operator, MySQL explicitly checks for records whose CODE column is NULL, ensuring that they are included in the result set along with records whose CODE is not equal to 'C'.

MySQL's behavior in this situation can be attributed to the way it treats NULL as an unknown or undefined value. Inequality comparisons, such as !=, return true only if there is a clear difference between the operands. Since NULL is an unknown value, it is not considered different from any other value (including 'C').

Therefore, when using inequality comparisons, be sure to explicitly account for NULL values ​​to ensure accurate results. The IS NULL operator provides a reliable way to handle this situation and ensures that all relevant data is returned.

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