JOIN duplicates occur when one row from one table matches multiple rows in another table. The resulting rows are repeated according to the relationship between the tables.
Unexpected duplicates can produce incorrect counts, inflated totals, and misleading business reports.
A customer with five orders will appear five times when customer information is joined directly to all orders.
SELECT
c.CustomerID,
c.CustomerName,
o.OrderID
FROM Customers c
INNER JOIN Orders o
ON c.CustomerID = o.CustomerID;Important in sales reporting, customer analysis, financial calculations, and dashboard development.
- One-to-many relationships naturally produce multiple rows.
- Duplicates are not always errors.
- Check the relationship between tables.
- Use DISTINCT only when unique results are actually required.
- Aggregation may be needed to return one row per entity.
Why do JOINs create duplicate rows?
A row can match multiple rows in the related table, causing the parent information to appear multiple times.
Are duplicate JOIN rows always an error?
No. They may be the expected result of a one-to-many or many-to-many relationship.