JOIN duplicates occur when one row from one table matches multiple rows in another table. The resulting query correctly returns multiple combinations, but they may appear duplicated from a business perspective.
Unexpected duplicate rows can produce incorrect counts, totals, and dashboard metrics.
One customer may have ten orders, so joining Customers to Orders produces ten rows for that customer.
SELECT
c.CustomerID,
c.CustomerName,
o.OrderID
FROM Customers c
INNER JOIN Orders o
ON c.CustomerID = o.CustomerID;Important for sales reporting, customer analytics, financial aggregation, and dashboard development.
- JOINs can legitimately produce multiple rows.
- One-to-many relationships commonly create multiple matches.
- Duplicate-looking rows are not always SQL errors.
- DISTINCT should not be used blindly.
- Check the relationship cardinality before fixing duplicates.
Why can a JOIN create duplicate rows?
A row can match multiple related rows in the other table, especially in one-to-many relationships.
Should DISTINCT always be used to remove duplicates?
No. DISTINCT can hide an underlying JOIN or data-model issue and should only be used when duplicate result rows are genuinely unnecessary.