JOIN duplicates can be handled by correcting JOIN conditions, aggregating related rows, using EXISTS, or using DISTINCT when the business requirement truly calls for unique combinations.
Removing duplicates incorrectly can hide data problems or produce incomplete results, so the underlying relationship must be understood first.
Return one row per customer with the total number of orders instead of one row for every order.
SELECT
c.CustomerID,
c.CustomerName,
COUNT(o.OrderID) AS OrderCount
FROM Customers c
LEFT JOIN Orders o
ON c.CustomerID = o.CustomerID
GROUP BY
c.CustomerID,
c.CustomerName;Used in customer reporting, KPI dashboards, financial reports, and data analysis.
- Correct JOIN conditions first.
- Aggregation can reduce detail rows.
- EXISTS can avoid duplicate parent rows.
- DISTINCT removes duplicate result combinations.
- Always identify why duplicates occur before removing them.
What is the best way to handle JOIN duplicates?
First understand the relationship and business requirement, then use the appropriate JOIN, aggregation, EXISTS, or DISTINCT approach.
Why should DISTINCT not be used blindly?
It can hide an incorrect JOIN or legitimate multiple records instead of solving the underlying problem.