SQL WHERE Clause: Using IS NULL
Quick Answer
The SQL WHERE clause with IS NULL is used to filter records where a column contains NULL values. It helps identify missing or undefined data in a table, which cannot be checked using standard equality operators.
Learning Objectives
- Understand what NULL means in SQL and why it requires special handling.
- Learn how to use the WHERE clause with IS NULL to filter records.
- Practice writing SQL queries that correctly identify NULL values.
Introduction
In SQL, NULL represents missing or undefined data in a table column.
Since NULL is not a value but a marker for absence of data, it requires special handling in queries.
The WHERE clause with IS NULL allows you to filter rows where a column contains NULL values.
"NULL is not a value, it is the absence of a value."
Understanding NULL in SQL
NULL indicates that a data value does not exist in the database. It is different from zero or empty string.
Because NULL is not a value, you cannot use standard comparison operators like '=' or '<>' to test for NULL.
Special operators like IS NULL and IS NOT NULL are used to check for NULL values.
- NULL means unknown or missing data.
- NULL is not equal to anything, even another NULL.
- Use IS NULL to find NULL values.
- Use IS NOT NULL to find non-NULL values.
Using WHERE Clause with IS NULL
The WHERE clause filters rows based on specified conditions.
To find rows where a column is NULL, use the syntax: WHERE column_name IS NULL.
This condition returns all rows where the column has no value.
- Syntax: SELECT * FROM table WHERE column IS NULL;
- IS NULL checks for NULL values explicitly.
- Cannot use '=' or '<>' to check for NULL.
Example Query
Suppose you have a table named Employees with a column called ManagerID that can be NULL if the employee has no manager.
To find employees without a manager, you write:
- SELECT * FROM Employees WHERE ManagerID IS NULL;
Common Use Cases for IS NULL
Filtering NULL values is useful in many scenarios such as data cleaning, reporting, and conditional logic.
You might want to find missing data to update it or exclude NULLs from calculations.
- Identify records missing important information.
- Exclude NULLs from aggregates or calculations.
- Join tables where foreign keys might be NULL.
- Implement conditional logic based on presence or absence of data.
Practical Example
This query returns all customers who do not have an email address recorded.
This query finds orders that have not yet been shipped because the ShipDate is NULL.
Examples
SELECT CustomerID, CustomerName FROM Customers WHERE Email IS NULL;This query returns all customers who do not have an email address recorded.
SELECT OrderID, OrderDate FROM Orders WHERE ShipDate IS NULL;This query finds orders that have not yet been shipped because the ShipDate is NULL.
Best Practices
- Always use IS NULL or IS NOT NULL to check for NULL values.
- Avoid using '=' or '<>' operators with NULL as they will not work as expected.
- Use NULL checks to improve data quality and handle missing data explicitly.
- Combine IS NULL with other conditions using AND/OR for complex filters.
Common Mistakes
- Using '=' or '<>' to compare with NULL values.
- Assuming NULL means zero or empty string.
- Ignoring NULL values in aggregate functions without proper handling.
- Not considering NULLs in JOIN conditions leading to unexpected results.
Hands-on Exercise
Find Records with NULL Values
Write a query to find all employees in the Employees table where the 'PhoneNumber' column is NULL.
Expected output: A list of employees missing phone numbers.
Hint: Use the WHERE clause with IS NULL.
Filter Non-NULL Records
Write a query to select all products from the Products table where the 'DiscontinuedDate' is NOT NULL.
Expected output: Products that have a discontinued date.
Hint: Use IS NOT NULL in the WHERE clause.
Interview Questions
Why can't you use '=' to check for NULL in SQL?
InterviewBecause NULL represents unknown or missing data, it is not equal to any value, including itself. Therefore, '=' does not work for NULL comparisons; IS NULL must be used instead.
What is the difference between IS NULL and IS NOT NULL?
InterviewIS NULL checks if a column contains NULL values, while IS NOT NULL checks for columns that have any non-NULL value.
How does SQL treat NULL in logical expressions?
InterviewIn SQL, NULL propagates through logical expressions resulting in UNKNOWN rather than TRUE or FALSE, which affects filtering and requires special handling.
MCQ Quiz
1. What is the best first step when learning IS NULL?
A. Understand the purpose and basic idea
B. Skip directly to advanced implementation
C. Ignore examples and practice
D. Memorize terms without context
Correct answer: A
Starting with the purpose and basic idea makes later examples and practice easier to understand.
2. Which activity helps reinforce IS NULL?
A. Reading once without practice
B. Building or writing a small practical example
C. Avoiding review questions
D. Skipping the summary
Correct answer: B
A small practical example helps connect the topic to real usage.
3. Which statement is most accurate about this topic?
A. The SQL WHERE clause with IS NULL is used to filter records where a column contains NULL values.
B. IS NULL never needs examples
C. IS NULL is unrelated to practical work
D. IS NULL should be learned without checking results
Correct answer: A
The correct option is based on the available topic explanation.
Key Takeaways
- NULL represents missing or unknown data in SQL.
- IS NULL is the correct way to check for NULL values in a WHERE clause.
- Using '=' or '<>' operators does not work for NULL comparisons.
- Filtering NULLs helps in data quality checks and conditional logic.
- The SQL WHERE clause with IS NULL is used to filter records where a column contains NULL values.
Summary
The SQL WHERE clause with IS NULL is essential for filtering rows with missing or undefined data.
NULL values require special handling because they represent absence of data, not a value.
Using IS NULL and IS NOT NULL correctly helps maintain data integrity and accurate query results.
Frequently Asked Questions
What does NULL mean in SQL?
NULL means that a data value is missing or unknown in a database column.
Can I use '=' to check for NULL values?
No, '=' does not work for NULL comparisons. You must use IS NULL or IS NOT NULL.
How do I find rows where a column is not NULL?
Use the WHERE clause with IS NOT NULL to filter rows where the column has a value.
