Null,Is Null, Is Not Null Operator in SQL

Table of Contents
1. Introduction
2. What is Null?
3. The IS NULL Operator
4. The IS NOT NULL Operator
5. Combining Null Operators with Other Conditions
6. Common Mistakes and Tips
7. Sample Problems and Solutions
8. Conclusion
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Introduction

When working with databases, you often encounter situations where a column value is unknown or missing. In SQL, the concept of null represents such missing or unknown values. To effectively handle null values in your queries, it is essential to understand the null, IS NULL, and IS NOT NULL operators. This article aims to provide a beginner-friendly explanation of these operators along with examples and sample problems to solidify your understanding.

What is Null?

Null represents the absence of a value or the unknown state of a data element in a database. It is different from an empty string or zero. A column with a null value indicates that the data is missing or has not been recorded.

The IS NULL Operator

The IS NULL operator is used to check whether a column contains a null value. It returns true if the value is null, and false otherwise.
Example 1: Retrieving rows with null values
Consider a "users" table with the following columns: "id," "name," and "email." To retrieve all users with null email addresses, you can use the IS NULL operator as follows:

SELECT * FROM users WHERE email IS NULL;

Code Explanation: The query selects all columns from the "users" table where the "email" column is null.

Output

id	name	email
1	John Doe	NULL
3	Jane Roe	NULL

The IS NOT NULL Operator

The IS NOT NULL operator is used to check whether a column contains a non-null value. It returns true if the value is not null, and false otherwise.
Example 2: Filtering rows with non-null values
Using the same "users" table, let's retrieve all users with non-null email addresses:

SELECT * FROM users WHERE email IS NOT NULL;

Code Explanation: The query selects all columns from the "users" table where the "email" column is not null.

Output

id	name	email
2	Alice Foo	alice@example.com

Combining Null Operators with Other Conditions

You can combine the null operators with other conditions using logical operators like AND and OR to create more complex queries.

Example 3: Combining null and non-null conditions
Let's retrieve all users with a null email address or whose name starts with "J":

SELECT * FROM users WHERE email IS NULL OR name LIKE 'J%';

Code Explanation: The query selects all columns from the "users" table where the "email" column is null or the "name" column starts with 'J'.

Output

id	name	email
1	John Doe	NULL
3	Jane Roe	NULL

Common Mistakes and Tips

• Null values cannot be compared directly using the equality operator (=). Always use IS NULL or IS NOT NULL to check for null values.
• When using IS NULL or IS NOT NULL, make sure to include them in parentheses when combining with other conditions to ensure the correct logical evaluation.

Sample Problems and Solutions

Problem 1: Retrieve all products with a non-null price from a "products" table.

SELECT * FROM products WHERE price IS NOT NULL;

Problem 2: Retrieve all orders with a null discount or a total greater than 100.

SELECT * FROM orders WHERE discount IS NULL OR total > 100;

Conclusion

Understanding null, IS NULL, and IS NOT NULL operators is crucial for effectively handling missing or unknown values in SQL queries. By using these operators, you can filter and retrieve the desired data based on null conditions. Remember to combine null operators with other conditions to create more complex queries. With practice and a solid understanding of these concepts, you'll be better equipped to work with null values in your database queries.