👉 “Why Your SQL LEFT JOIN Is Creating Duplicate Rows (And How to Fix It Fast)”

 

🚨 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.

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🔍 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

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💥 Your Query

SELECT *
FROM Customers A
LEFT JOIN Orders B
ON A.CustomerID = B.CustomerID

👉 Output:

• Vijay appears 2 times (because 2 orders exist)

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⚠️ Real Issue

SQL is working correctly ❗
But your expectation is wrong.

👉 JOIN multiplies rows when matching multiple records.

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✅ 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)

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✅ 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

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✅ 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

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🔥 Pro Tip (Very Important)

Before JOIN, always ask:
👉 “Is this one-to-one or one-to-many?”

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💡 Real-World Example

This issue happens in:

• Campaign data (your case 😉)

• Invoice joins

• Employee + attendance data

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✅ Final Thoughts

If your SQL is returning duplicate rows:
👉 Don’t panic
👉 Check relationship
👉 Apply correct fix