🧺 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

✅ Recommended	❌ Avoid This
Use IS NULL/IS NOT NULL	Using = NULL or <> NULL
Replace NULLs in output using COALESCE()	Allowing NULLs to break UI logic
Treat NULLs carefully in joins	Assuming NULL = 0 or ”

---

📌 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.

---

Share Now :