Beginner

472. JOIN with NULL Check

Learn how NULL checks can identify unmatched records after an OUTER JOIN.

📘 Lesson 472 of 728 ⏱ 11 min 🎯 Intermediate

NULL checks using IS NULL or IS NOT NULL can identify whether a matching record exists after an OUTER JOIN.

Checking NULL values after a JOIN is a simple and powerful way to find missing relationships between tables.

Find customers who have never placed an order.

SQL 
SELECT
    c.CustomerID,
    c.CustomerName
FROM Customers c
LEFT JOIN Orders o
    ON c.CustomerID = o.CustomerID
WHERE o.OrderID IS NULL;

Used for customer inactivity analysis, reconciliation, missing-data checks, and data-quality validation.

  • IS NULL can identify unmatched rows.
  • LEFT JOIN preserves all left-side records.
  • NULL checks are useful for finding missing relationships.
  • Do not use = NULL.
  • IS NOT NULL identifies available matching values.
How can LEFT JOIN find customers without orders?

Use LEFT JOIN and filter the right-side key with IS NULL.

Why should you use IS NULL instead of = NULL?

NULL represents an unknown value, so SQL uses IS NULL and IS NOT NULL for NULL comparisons.

Have you finished this lesson?

Introduction to SQL – All Lessons