IS NULL vs. = NULL in SQL: What's the Correct Way to Test for Null Values?

Deeply explore the difference between IS NULL and = NULL in SQL

Filtering rows based on the presence or absence of NULL values ​​is a common task in SQL. Understanding the nuances of testing for NULL values ​​using IS NULL and = NULL is critical to writing accurate queries.

IS NULL: Explicit NULL value test

IS NULL is a SQL operator specifically designed to test whether a field contains a NULL value. Unlike value operators such as =, the result of IS NULL is TRUE when the field is NULL and FALSE otherwise.

= NULL: not a true equivalence test

Unlike IS NULL, field = NULL is an equality comparison that attempts to compare the value of the field to a NULL literal. In SQL, a NULL value compares to any other value (even another NULL) to NULL itself. According to SQL logic, NULL values ​​are inherently unknown, so direct equality comparisons are unreliable.

The impact of filtering

In a WHERE clause, a condition containing field = NULL will always evaluate to NULL, which is considered FALSE in SQL. Therefore, field = NULL cannot properly filter rows containing NULL values. For this purpose, field IS NULL should be used.

Example

Consider the following SQL statement:

<code class="language-sql">SELECT * FROM table WHERE column = NULL;</code>

This statement will not return any rows even if the column field contains a NULL value. To correctly filter NULL values, the correct statement would be:

<code class="language-sql">SELECT * FROM table WHERE column IS NULL;</code>

Usage Guide

Although IS NULL and = NULL look similar, it is important to remember that IS NULL is the preferred and correct way to test NULL values ​​in SQL queries. Due to SQL's unique handling of NULL values, using = NULL can lead to misleading results. By following the correct query methodology, developers can ensure the accuracy and efficiency of their SQL code.

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