SQL NULL Values – Meaning, Syntax, and Best Practices

Introduction – Why NULL Matters in SQL

In SQL, NULL represents a missing, undefined, or unknown value. It does not mean zero, blank, or empty — rather, it means that a value is absent altogether.

Understanding how to handle NULL is crucial for:

• Accurate filtering and reporting
• Consistent conditional logic
• Avoiding incorrect query results or errors

In this guide, you’ll learn:

• What NULL means in SQL
• How to test for NULL using IS NULL and IS NOT NULL
• Use of NULL with comparison and logical operators
• Handling NULL in expressions and functions

1. What is a NULL Value?

A NULL in SQL indicates no value assigned to a column.

INSERT INTO students (name, age) VALUES ('John', NULL);

age is unknown or not provided for this student.

2. How to Check for NULL Values

Use IS NULL or IS NOT NULL to test for NULLs.

SELECT * FROM employees WHERE manager_id IS NULL;
SELECT * FROM employees WHERE manager_id IS NOT NULL;

Never use = NULL or != NULL — they won’t work.

3. NULLs in Conditions – Unexpected Results

SELECT * FROM students WHERE grade != 'A';

This query ignores rows where grade is NULL, because NULL != 'A' is unknown.

4. NULL in Expressions and Aggregates

• NULL + 10 → NULL
• AVG(NULL) → NULL
• COUNT(column) ignores NULLs
• COUNT(*) includes NULLs

Use COALESCE() or ISNULL() to replace NULLs:

SELECT name, COALESCE(phone, 'N/A') FROM contacts;

5. Handling NULL in ORDER BY

SELECT name FROM students ORDER BY grade ASC;

NULLs typically appear last (or first, depending on the RDBMS).

6. Using CASE for NULL Control

SELECT name,
  CASE
    WHEN grade IS NULL THEN 'Pending'
    ELSE grade
  END AS final_grade
FROM students;

Great for readable reports and dashboards.

Best Practices

Summary – Recap & Next Steps

NULL values are fundamental to how SQL handles missing or incomplete data. Properly checking, handling, and replacing them leads to more reliable and meaningful queries.

Key Takeaways:

• NULL means “no value” — not zero or empty
• Use IS NULL, IS NOT NULL to filter
• COUNT(column) ignores NULLs; COUNT(*) includes them
• Use COALESCE() or CASE for default output handling

Real-World Relevance:
Used in form submissions, incomplete data processing, optional fields, and ETL workflows.

Next: Learn how to handle SQL DEFAULT values and column constraints.

FAQ – SQL NULL Values

What is a NULL in SQL?

A NULL is a special marker that means the value is missing or unknown.

Can I compare NULL with = or !=?

No. Always use IS NULL or IS NOT NULL.

Does COUNT() include NULLs?

COUNT(column) ignores NULLs. Use COUNT(*) to count all rows.

How do I replace NULL with default text?

Use COALESCE(column, 'Default') or ISNULL(column, 'Default') (SQL Server).

Are NULLs sorted first or last?

It depends on the RDBMS and sort order. Most put NULLs last in ascending order.