SQL Null Comparisons: Why Use `IS NULL` Instead of `=`?

The problem of SQL null value comparison: =NULL and IS NULL

In the SQL world, null values ​​pose unique challenges to data manipulation and retrieval. Understanding the difference between "x is null" and "x = null" syntax is critical to effective query optimization.

Syntax and Semantics

The "x is null" expression explicitly tests whether the field "x" is null using the IS NULL operator. This operator returns a Boolean value, "true" if the field is empty and "false" otherwise.

In contrast, the "x = null" expression uses the equal sign comparison operator (=) to compare the value in field "x" to the literal null value. However, this comparison does not produce the expected Boolean result.

Null value comparison logic

According to SQL semantics, any comparison involving a null value, including an equality comparison with a null value, will return the null value itself. This is because a null value represents an unknown value, making any comparison undetermined. Therefore, the "x = null" expression will always return a null value, regardless of the actual value in field "x".

The

IS NULL operator overcomes this limitation by providing a syntax specifically designed for null testing. It allows developers to explicitly verify whether a field is empty, enabling precise querying and filtering operations.

Precautions for usage

In practical applications, the IS NULL operator is more suitable for explicit null value comparison. Using the "=" operator with null values ​​may cause unexpected results and may affect the accuracy of your query. It is recommended to use "IS NULL" instead of "=" in SQL statements to test the null value of a field.

To illustrate the difference between these two syntaxes, consider the following SQL query:

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

This query only retrieves rows where the "x" field is explicitly empty. If we use the "=" operator instead:

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

The query will not return any results even if there is a record with a null value in the "x" field. This is because the "x = NULL" comparison always evaluates to NULL, which is treated as "false" in the WHERE clause calculation.

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