How Do `!=` and `` Operators Behave with NULL Values in SQL?

SQL's != and <> Operators and NULL Values: A Clarification

SQL's != (not equal to) and <> operators exhibit unique behavior when encountering NULL values. Understanding this requires grasping the essence of NULL in SQL.

NULL signifies the absence of a value, not an empty string ("") or zero (0). It indicates that a field or column hasn't been assigned a data value.

Normally, != and <> return true if values differ. However, this doesn't apply to NULL.

Consider this:

<code class="language-sql">SELECT * FROM MyTable WHERE MyColumn != NULL; -- (0 Results)</code>

This query yields zero results, even if MyColumn contains non-NULL entries. The reason? != and <> cannot operate on NULL. They need a concrete value for comparison.

Conversely:

<code class="language-sql">SELECT * FROM MyTable WHERE MyColumn IS NOT NULL; -- (568 Results)</code>

This query correctly returns all rows where MyColumn is not NULL. IS NOT NULL is specifically designed to check for the absence of a value (NULL).

In summary: NULL is a unique value in SQL. Using != or <> to compare against NULL always results in false. To accurately test for the absence of a value, employ the IS NOT NULL operator.

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