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449. JOIN and Duplicate Rows

Understand why JOINs can create duplicate-looking rows and how to identify the cause.

📘 Lesson 449 of 728 ⏱ 12 min 🎯 Intermediate

JOINs can return multiple rows for a single record when the relationship between tables is one-to-many or many-to-many.

Unexpected row multiplication can produce incorrect counts, totals, averages, and reports.

A customer with three orders will appear three times when customer data is joined directly to order records.

SQL 
SELECT
    c.CustomerID,
    c.CustomerName,
    o.OrderID
FROM Customers c
INNER JOIN Orders o
    ON c.CustomerID = o.CustomerID;

Understanding join duplication is essential for sales reporting, financial analysis, reconciliation, and dashboard accuracy.

  • One-to-many relationships naturally produce multiple rows.
  • Duplicate-looking rows are not always actual duplicates.
  • Check the relationship between joined tables.
  • Aggregation may be required after the JOIN.
  • Incorrect joins can multiply rows unexpectedly.
Why can a JOIN create duplicate rows?

A JOIN can return multiple matching rows when one record has multiple related records.

How can you investigate unexpected duplicates?

Check the join keys, relationship cardinality, duplicate key values, and whether the JOIN condition is complete.

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