SQL NULL Values

NULL in SQL means no value or unknown. It is not the same as zero or an empty string. When a column has no data entered for a particular row, it stores NULL.

Think of NULL like a blank field on a form — the person did not fill it in. It is not zero, it is simply missing.

The Reference Table

Checking for NULL with IS NULL

You cannot use = NULL to check for NULL values. You must use IS NULL.

MySQL Query

-- Find students where City is missing
SELECT * FROM Students WHERE City IS NULL;

MS SQL Server Query

-- Find students where City is missing
SELECT * FROM Students WHERE City IS NULL;

Checking for NOT NULL

MySQL Query

-- Find students where City has a value
SELECT * FROM Students WHERE City IS NOT NULL;

MS SQL Server Query

-- Find students where City has a value
SELECT * FROM Students WHERE City IS NOT NULL;

Replacing NULL with a Default Value

Sometimes you want to display a default value instead of NULL in the output. The function name differs between MySQL and MS SQL Server.

MySQL Query

-- IFNULL replaces NULL with 'Not Provided' in MySQL
SELECT StudentName, IFNULL(City, 'Not Provided') AS City
FROM Students;

MS SQL Server Query

-- ISNULL replaces NULL with 'Not Provided' in MS SQL Server
SELECT StudentName, ISNULL(City, 'Not Provided') AS City
FROM Students;

COALESCE — Works in Both Databases

COALESCE is the ANSI standard function that works the same in both MySQL and MS SQL Server. It returns the first non-NULL value from a list.

MySQL Query

SELECT StudentName, COALESCE(City, 'Not Provided') AS City
FROM Students;

MS SQL Server Query

SELECT StudentName, COALESCE(City, 'Not Provided') AS City
FROM Students;

Key Points

• NULL means "no value" — it is not zero, not an empty string, just unknown.
• Use IS NULL and IS NOT NULL — never use = NULL.
• MySQL uses IFNULL(column, default) to replace NULL with a default value.
• MS SQL Server uses ISNULL(column, default) — the same purpose, different name.
• COALESCE(column, default) is the ANSI standard and works in both databases.