SQL NULL Handling

🟡 SQL NULL Handling

In SQL, NULL represents a missing or unknown value. It is not the same as 0 or an empty string ''. Handling NULL correctly is essential for accurate results.

1️⃣ Identifying NULL Values

To check if a column has NULL values, use IS NULL:

SELECT FirstName, ManagerID
FROM Employees
WHERE ManagerID IS NULL;

👉 Finds employees who don’t have a manager assigned.

2️⃣ Filtering Non-NULL Values

To check if a column is not null, use IS NOT NULL:

SELECT FirstName, ManagerID
FROM Employees
WHERE ManagerID IS NOT NULL;

👉 Finds employees who have a manager assigned.

3️⃣ NULL in Comparisons

Important: NULL does not equal anything, not even another NULL. This means:

SELECT * 
FROM Employees
WHERE ManagerID = NULL; -- ❌ This will never work

✅ Always use IS NULL or IS NOT NULL instead.

4️⃣ Using NULL with Functions

Functions can handle NULL differently:

SELECT AVG(Salary), SUM(Salary)
FROM Employees;

👉 NULL salaries are ignored in aggregate functions like SUM, AVG, COUNT.

SELECT FirstName, COALESCE(ManagerID, 'No Manager') AS ManagerStatus
FROM Employees;

✅ Key Takeaways

• NULL = unknown / missing value.
• Use IS NULL / IS NOT NULL to check NULL values.
• NULL ≠ NULL (they are never equal).
• Use COALESCE to replace NULL with default values.

📚 Continue Learning