SQL NULL Equality: When to Use IS NULL vs. = NULL?

The paradox of SQL NULL value comparison: distinguishing between IS NULL and = NULL

In the field of SQL, the concept of NULL values ​​often brings challenges to developers. One of the difficulties is when using the IS NULL and = NULL operators to compare NULL values. Even though they look similar, their results are very different, confusing many people.

Understand the differences

The result of
• = NULL is unknown, which represents three-valued logic. However, in the WHERE clause, this result is treated as false, excluding rows with NULL values ​​in the result set.
• IS NULL Explicitly checks for NULL values ​​and returns true only if the value is indeed NULL.

When to use each operator

Given their different behaviors:

• When you want to intentionally search for unknown values, use = NULL. Not recommended for general comparisons as it may lead to unexpected results.
• When you need to specifically check for NULL values, use IS NULL. This is the preferred way to handle NULL comparisons in SQL.

Example

Consider the following form:

<code>| id | name |
|---|---|
| 1  | John |
| 2  | NULL |
| 3  | Mary |</code>

• Query 1:

<code>SELECT * FROM table WHERE name = NULL;</code>

Result: No rows are returned because = NULL treats NULL as false.

• Query 2:

<code>SELECT * FROM table WHERE name IS NULL;</code>

Result: Returns a row with id 2, correctly identifying NULL values.

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