How to Compare Two Values When One is NULL

In SQL-based platforms like Snowflake, handling NULL values during comparisons can be tricky. NULL represents an unknown value, so comparing a NULL value with any other value (including another NULL) doesn't return the expected result. Understanding how to handle these cases can make your queries more robust and error-free.

Understanding NULL in SQL

In SQL, NULL is not the same as an empty string or zero. It signifies the absence of a value, and it is not comparable to any other value directly. Therefore, when you try to compare a NULL value to another value, SQL doesn't return a true or false result as you might expect.

Comparing NULL in Snowflake

To compare two values when one or both are NULL, Snowflake provides special handling for NULL values. Let’s break it down with some examples:

1. Using IS NULL or IS NOT NULL

Instead of using the standard comparison operators (like '=', '>', or '<'), Snowflake requires the use of IS NULL and IS NOT NULL for checking NULL values.

SELECT *
FROM table_name
WHERE column_name IS NULL;
                                

2. Using COALESCE to Replace NULL

If you want to compare a value and handle NULLs gracefully, COALESCE can be helpful. It returns the first non-NULL expression in a list of arguments. For instance:

SELECT *
FROM table_name
WHERE COALESCE(column_name, 'default_value') = 'desired_value';
                                

In this example, if column_name is NULL, it will be replaced by the string 'default_value' for the comparison.

3. Handling NULL in Equality Comparisons

Equality checks between NULL values will not work as expected because NULL is not equal to NULL. To check for equality while considering NULLs, use the IS DISTINCT FROM operator in Snowflake:

SELECT *
FROM table_name
WHERE column_name IS DISTINCT FROM other_column;
                                

This will return the rows where the values are different, even if one or both are NULL.

Best Practices for Handling NULL in Snowflake

• Always use IS NULL or IS NOT NULL for null value checks.
• Use COALESCE or NVL to replace NULL values when necessary.
• For equality comparisons that involve NULL, use IS DISTINCT FROM instead of the regular comparison operators.

By understanding and using these methods, you can avoid errors and ensure your queries behave as expected, even when NULL values are involved.