🚨 Problem (Relatable Hook)
You wrote a simple LEFT JOIN…
But suddenly your data is duplicated 😵
👉 Example:
-
Expected: 100 rows
-
Got: 500 rows
Sound familiar? This is one of the most common SQL mistakes.
🔍 Why This Happens
The main reason is:
👉 One-to-Many relationship
Example:
Table A (Customers)
| CustomerID | Name |
|---|---|
| 1 | Vijay |
Table B (Orders)
| CustomerID | OrderID |
|---|---|
| 1 | 101 |
| 1 | 102 |
💥 Your Query
SELECT *
FROM Customers A
LEFT JOIN Orders B
ON A.CustomerID = B.CustomerID
👉 Output:
-
Vijay appears 2 times (because 2 orders exist)
⚠️ Real Issue
SQL is working correctly ❗
But your expectation is wrong.
👉 JOIN multiplies rows when matching multiple records.
✅ Fix #1 – Use DISTINCT
SELECT DISTINCT A.CustomerID, A.Name
FROM Customers A
LEFT JOIN Orders B
ON A.CustomerID = B.CustomerID
👉 Removes duplicates (quick fix)
✅ Fix #2 – Use Aggregation (Best Practice)
SELECT
A.CustomerID,
A.Name,
COUNT(B.OrderID) AS TotalOrders
FROM Customers A
LEFT JOIN Orders B
ON A.CustomerID = B.CustomerID
GROUP BY A.CustomerID, A.Name
👉 Clean + meaningful result
✅ Fix #3 – Join with Filter
SELECT *
FROM Customers A
LEFT JOIN (
SELECT CustomerID, MAX(OrderID) AS OrderID
FROM Orders
GROUP BY CustomerID
) B ON A.CustomerID = B.CustomerID
👉 Returns only one row per customer
🔥 Pro Tip (Very Important)
Before JOIN, always ask:
👉 “Is this one-to-one or one-to-many?”
💡 Real-World Example
This issue happens in:
-
Campaign data (your case 😉)
-
Invoice joins
-
Employee + attendance data
✅ Final Thoughts
If your SQL is returning duplicate rows:
👉 Don’t panic
👉 Check relationship
👉 Apply correct fix
