How to Correctly Compare NULL Values in SQL Queries?

In-depth understanding of SQL NULL value comparison

In SQL, the NULL value represents an unknown or empty value. When comparing NULL to other values ​​using regular comparison operators (e.g. =, !=), the result is always NULL. This can cause unexpected behavior in query results.

The difference between “where x is null” and “where x = null”

The "where x is null" syntax explicitly checks whether column x is NULL, while "where x = null" attempts to compare the column value to a NULL literal. The latter comparison fails because it results in NULL, which is always false in the WHERE clause.

Explanation of NULL value behavior

SQL interprets NULL as "unknown", which means that any comparison to NULL will result in an unknown state. Therefore, regular comparison operators cannot effectively test NULL values.

IS NULL and IS NOT NULL syntax

To solve this problem, SQL provides "IS NULL" and "IS NOT NULL" syntax. These special conditions explicitly check for the presence of NULL values ​​in the column. "IS NULL" returns true if the column is NULL, while "IS NOT NULL" returns true if the column is not NULL.

Example

The following SQL query illustrates the difference between regular comparison operators and IS NULL syntax:

<code class="language-sql">CREATE TABLE example (value INT);
INSERT INTO example VALUES (1), (NULL);

SELECT * FROM example WHERE value = NULL; -- 返回空结果
SELECT * FROM example WHERE value IS NULL; -- 返回包含NULL值的行</code>

This query demonstrates that regular comparison does not return rows containing NULL values, while the IS NULL syntax correctly identifies it.

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