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450. JOIN and Duplicate Data

Learn how duplicate values in join columns can affect SQL results.

📘 Lesson 450 of 728 ⏱ 12 min 🎯 Intermediate

Duplicate values in join columns can cause multiple matches between tables, increasing the number of rows returned by the query.

Duplicate join keys can lead to inflated totals and incorrect analytical results.

If a customer ID appears twice in a customer reference table, joining it to orders can duplicate the corresponding order records.

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

This issue is important in data quality checks, financial reconciliation, customer reporting, and ETL processes.

  • Duplicate join keys can multiply result rows.
  • Join-key uniqueness should be understood.
  • Data quality affects JOIN results.
  • Duplicates can inflate aggregates.
  • Use appropriate keys and validation checks.
How do duplicate join keys affect results?

They can cause one row to match multiple rows and therefore multiply the result set.

Why should join-key uniqueness be checked?

Checking uniqueness helps prevent unexpected row multiplication and incorrect calculations.

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