Why Do '!=' and '<>' Operators Fail to Compare with NULL in SQL?

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Understanding NULL Comparisons in SQL

SQL uses "!=" and "<>" for inequality checks. However, these operators behave unexpectedly with NULL values.

The Issue:

Consider these SQL queries:

<code class="language-sql">SELECT * FROM MyTable WHERE MyColumn != NULL;
SELECT * FROM MyTable WHERE MyColumn <> NULL;
SELECT * FROM MyTable WHERE MyColumn IS NOT NULL;</code>

The first two queries return empty result sets, whereas the third correctly returns 568 rows. Why this discrepancy?

Explanation:

"!=" and "<>" compare values. NULL, however, isn't a value; it signifies the absence of a value. Therefore, these operators are unsuitable for NULL comparisons.

Use IS NULL or IS NOT NULL instead. These predicates specifically check for the presence or absence of a value. The third query's success stems from its correct use of IS NOT NULL.

Key Points:

• To test for non-NULL values, use IS NOT NULL.
• Use "<>" or "!=" to compare against specific non-NULL values.
• NULL cannot be considered equal to or unequal to any value, as it represents missing data.

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