Understanding “Is Null” in SQL Server

Dear Dev, if you are working with SQL Server, you have probably come across the term “is null” at some point in your career. This term is often used in SQL queries to check whether a particular value is null or not. But what exactly does it mean? In this article, we will dive deep into the concept of “is null” in SQL Server and explore its various applications. So, let’s get started!

What is “Is Null” in SQL Server?

“Is null” is a logical operator in SQL Server that is used to check whether a value is null or not. In SQL, null represents the absence of a value or an unknown value. It is not the same as an empty string (”) or a zero value (0). When we use “is null” in a SQL query, it returns true if the value is null, and false if it is not.

For example, let’s say we have a table called “employees” with columns “id”, “name”, and “salary”. If we want to find out the names of all employees who haven’t received a salary yet, we can use the following query:

Query
Result
SELECT name FROM employees WHERE salary is null;
John, Jane

How to Use “Is Null” in SQL Server?

There are several ways to use “is null” in SQL Server. Let’s take a look at some common scenarios:

Checking for Null Values

If you want to check whether a value in a column is null or not, you can use the “is null” operator in the WHERE clause of your SQL query. For example, if you want to find all the records where the “salary” column is null, you can use the following query:

Query Result
SELECT * FROM employees WHERE salary is null; 1 John NULL 2 Jane NULL

Checking for Non-Null Values

Conversely, if you want to find all the records where the “salary” column is not null, you can use the “is not null” operator in your SQL query. For example:

Query Result
SELECT * FROM employees WHERE salary is not null; 3 Bob 50000 4 Mary 70000

Using “Is Null” in Subqueries

You can also use “is null” in subqueries to find records where a column is null in a related table. For example, let’s say we have a table called “departments” with columns “id” and “name”, and another table called “employees” with columns “id”, “name”, “salary”, and “dept_id”. If we want to find all the departments that don’t have any employees, we can use the following query:

Query
Result
SELECT name FROM departments WHERE id not in (SELECT dept_id FROM employees WHERE dept_id is not null);
HR

Using “Is Null” with Joins

You can also use “is null” with joins to find records where a column is null in one or more tables. For example, let’s say we have the same “departments” and “employees” tables as before, but this time we want to find all the departments that don’t have any employees with a salary greater than 50000. We can use the following query:

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Query
Result
SELECT d.name FROM departments d LEFT JOIN employees e ON d.id = e.dept_id AND e.salary > 50000 WHERE e.id is null;
IT

FAQs About “Is Null” in SQL Server

Q. Can “Is Null” be Used with Numeric Data Types?

A. Yes, “is null” can be used with numeric data types, such as INT, BIGINT, DECIMAL, and FLOAT. If a numeric value is null, “is null” will return true.

Q. What is the Difference Between “Is Null” and “IsNull”?

A. “Is null” is a logical operator in SQL Server, while “IsNull” is a built-in function that returns the specified value if the expression is null, and the expression itself otherwise.

Q. Can “Is Null” be Used with Dates and Times?

A. Yes, “is null” can be used with date and time data types, such as DATETIME, DATE, TIME, and DATETIME2. If a date or time value is null, “is null” will return true.

Q. Can I Use “Is Null” in an IF Statement?

A. Yes, you can use “is null” in an IF statement in SQL Server, like this:

Query
IF @variable is null BEGIN PRINT ‘Variable is null’; END

Q. Can “Is Null” Cause Performance Issues?

A. In general, “is null” is a fast and efficient operation in SQL Server. However, if you use it in a large dataset with complex joins and subqueries, it could potentially impact performance. As with any SQL operation, it’s important to optimize your queries and use indexes where necessary to ensure optimal performance.

Conclusion

Well done, Dev! You have now learned all about “is null” in SQL Server and how to use it in various scenarios. Whether you’re checking for null values, using it in subqueries or joins, or incorporating it into an IF statement, “is null” is a powerful tool in your SQL arsenal. As always, it’s important to practice and experiment with SQL queries to gain a deeper understanding of this powerful language. Happy coding!

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