How to Compare Two Values When One is NULL

In SQL Server, comparing two values where one of them might be NULL can be tricky due to the way NULL values are handled. In SQL, NULL is not considered equal to any other value, even another NULL. This can lead to unexpected results when using comparison operators like `=`, `!=`, `<`, or `>`. Let’s explore the methods to handle NULL values properly in your queries.

Using IS NULL

The `IS NULL` condition is used to check if a value is NULL. If you are comparing a column to another column or value and one of them might be NULL, you can use the `IS NULL` condition to explicitly check for NULL values.

SELECT *
FROM your_table
WHERE your_column IS NULL;

Using COALESCE for NULL Handling

The `COALESCE()` function can be used to replace NULL values with a specified value. This is useful when you want to avoid NULL values in your comparison.

SELECT *
FROM your_table
WHERE COALESCE(your_column, 'default_value') = 'desired_value';

In this example, if `your_column` is NULL, it will be replaced with the string `'default_value'` before making the comparison.

Using CASE Statements

Another way to handle NULL comparisons is by using a `CASE` statement. This allows you to define conditions that deal with NULL values explicitly and return the result accordingly.

SELECT CASE
                    WHEN your_column IS NULL THEN 'Value is NULL'
                    WHEN your_column = 'desired_value' THEN 'Match found'
                    ELSE 'No match'
                END as result
FROM your_table;

The `CASE` statement in the above query returns different values based on whether `your_column` is NULL or equals the desired value.

Conclusion

Handling NULL values in SQL Server requires specific techniques to ensure accurate comparisons. Using `IS NULL`, `COALESCE()`, or `CASE` allows you to explicitly define how NULL values should be treated in your queries. Understanding these techniques will help you avoid errors and get the desired results when working with NULL values in SQL Server.