Can We Compare Null Values in SQL?

Comparing values in SQL is straightforward, but it becomes tricky when dealing with NULL values. A NULL value represents the absence of a value, not a blank space or zero. Therefore, using standard comparison operators like = or != with NULL will always result in NULL, which is interpreted as false in conditional statements. So, how can we effectively compare NULL values in SQL?

Understanding NULL Comparison Behavior

The fundamental principle to grasp is that NULL represents an unknown value. You can’t definitively say if an unknown value is equal to, greater than, or less than another value, even if that other value is also NULL. This is why direct comparisons with NULL always yield NULL.

Let’s illustrate with an example. Consider a table named Employees with a column Bonus. If an employee hasn’t been awarded a bonus, the Bonus column might contain a NULL value. The following query wouldn’t return employees with a NULL bonus even though logically, we might want it to:

SELECT * FROM Employees WHERE Bonus = NULL; -- Returns no rows

Utilizing IS NULL and IS NOT NULL

To effectively check for NULL values, SQL provides specialized operators: IS NULL and IS NOT NULL.

IS NULL: This operator returns TRUE if the compared value is NULL.

SELECT * FROM Employees WHERE Bonus IS NULL; -- Returns employees with NULL bonus

IS NOT NULL: This operator returns TRUE if the compared value is not NULL.

SELECT * FROM Employees WHERE Bonus IS NOT NULL; -- Returns employees with a bonus value

These operators provide a reliable way to determine if a field lacks a value.

Comparing Two Potentially NULL Values

The challenge arises when comparing two fields, both of which might be NULL. You might need to consider both scenarios: when both are NULL (considered equal in some contexts) and when one is NULL and the other isn’t.

SELECT * 
FROM Employees 
WHERE column1 = column2 
   OR (column1 IS NULL AND column2 IS NULL); -- Handles both NULL scenarios

This query returns rows where column1 and column2 have the same value (including both being NULL).

Leveraging COALESCE for NULL Substitution

The COALESCE function offers an elegant solution for handling NULL comparisons. It returns the first non-NULL expression in a list. You can use it to provide a default value when a field is NULL.

SELECT * 
FROM Employees
WHERE column1 = COALESCE(column2, 0); -- Treats NULL in column2 as 0

This example compares column1 with column2, but if column2 is NULL, it compares column1 with 0. Choose the default value based on your specific comparison logic.

Using NULLIF for Specific Scenarios

The NULLIF function returns NULL if two expressions are equal. This is useful in specific situations, such as preventing division by zero errors.

SELECT column1 / NULLIF(column2, 0) AS result 
FROM YourTable;  -- Avoids division by zero

If column2 is 0, NULLIF(column2, 0) returns NULL, and the division results in NULL instead of an error.

Conclusion

Comparing NULL values in SQL requires a careful understanding of how NULL behaves. Avoid direct comparisons using = or !=. Instead, utilize the IS NULL and IS NOT NULL operators for direct NULL checks, or employ functions like COALESCE and NULLIF to manage NULL values within comparisons depending on your desired logic. By employing these techniques, you can accurately and efficiently handle NULL values in your SQL queries.