How Do SQL's `!=` and `` Operators Differ from `IS NOT NULL` When Handling NULL Values?

Handling NULLs with SQL's Inequality Operators: !=, <>, and IS NOT NULL

SQL's inequality operators (!= and <>) behave differently with NULL values compared to non-NULL values. This is because NULL represents the absence of a value, not a specific value.

!=/<> vs. IS NOT NULL: A Key Difference

Consider this query:

<code class="language-sql">SELECT * FROM MyTable WHERE MyColumn != NULL;</code>

This query will return an empty result set. The != (or <>) operator compares values, and NULL is not a value; the comparison is undefined.

In contrast:

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

This query returns all rows where MyColumn has a value (i.e., is not NULL). IS NOT NULL is specifically designed to check for the presence or absence of NULL.

Comparing with Non-NULL Values

The != and <> operators function as expected when comparing non-NULL values:

<code class="language-sql">SELECT * FROM MyTable WHERE MyColumn <> 'MyValue';</code>

This returns rows where MyColumn is different from 'MyValue'.

Summary

Remember: != and <> compare values, while IS NOT NULL checks for the absence of a value (NULL). Understanding this distinction is critical for accurate data filtering and manipulation in SQL.

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