Working with missing data is one of the most common challenges in database management, and knowing how to check IS NULL or empty in SQL is an essential skill for every developer, data analyst, and database administrator. Even so, whether you are cleaning customer records, validating form submissions, or building reports, distinguishing between a missing value and an intentionally blank field can determine whether your query returns accurate results or silently skips critical rows. This guide explains the difference between NULL and empty strings, shows you how to test for both conditions across major database systems, and provides practical functions to handle them safely in your SQL queries Easy to understand, harder to ignore. Took long enough..
Understanding NULL and Empty Values in SQL
Before writing queries, you must understand what NULL and empty actually mean in a relational database. These two concepts look similar in output, but they behave very differently under the hood.
What Is NULL in SQL?
NULL is a special marker that indicates missing, unknown, or not applicable data. It is not a value like zero or an empty string; rather, it represents the absence of a value. When a column allows NULL and no data is inserted, the database stores NULL to show that the information is unavailable.
Here's one way to look at it: if a customer record does not include a phone number because the customer never provided one, the phone_number column might store NULL. This tells you that the data is missing, not that the customer intentionally has no phone.
What Is an Empty String?
An empty string is a valid value with zero characters, written as '' in SQL. Even so, it means the field exists and has been explicitly set to contain nothing. If a user submits a form and leaves a text box blank, the application might insert an empty string into the database rather than NULL.
It is also important to distinguish an empty string from a string that contains only spaces, such as ' '. Technically, a whitespace string is not empty because it contains characters, but in many business scenarios it should be treated as blank.
How to Check for NULL Values Using IS NULL
SQL provides dedicated operators to test for NULL because standard comparison operators do not work the way most people expect.
The IS NULL Operator
To find rows where a column contains NULL, use the IS NULL operator:
SELECT customer_id, first_name, email
FROM customers
WHERE phone_number IS NULL;
This query returns every customer who does not have a phone number recorded. You cannot use = NULL because NULL is not equal to anything, including itself. In SQL’s three-valued logic, the result of NULL = NULL is UNKNOWN, not TRUE.
Quick note before moving on.
