Can JOINs cause duplicate rows in results? How do you eliminate them?
How JOINs Can Produce Duplicate Rows and How to Remove Them
Yes, JOINs can cause duplicate rows when the relationship between tables is not strictly one-to-one. Any one-to-many or many-to-many relationship will multiply rows, which may appear as duplicates in the result set—especially when joining tables like customers → orders → order_items.
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One-to-many relationships expand rows (e.g., one order with multiple payments).
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Duplicate matching values in the joined table multiply rows.
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Lack of proper join conditions results in unintended row combinations.
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Joining denormalized tables may bring repeated values that look like duplicates.
If a customer has 3 orders, they appear 3 times. These aren't true "duplicates"—they're correct representations of a one-to-many relationship. But sometimes duplicates are accidental and need removal.
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Use DISTINCT to return only unique combinations of selected columns.
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Use GROUP BY to collapse rows (only when grouping makes sense).
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Refine JOIN conditions to avoid unintended matches.
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Normalize data to prevent duplicate stored values.
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Use EXISTS instead of JOIN when only existence needs to be checked.
EXISTS is often faster and avoids row-multiplying JOINs when you're only checking whether related data exists.
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Understand the relationship (one-to-one, one-to-many, many-to-many).
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Use DISTINCT only when logically correct—it hides problems instead of fixing them.
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Use GROUP BY when aggregating data, not just to remove duplicates.
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Avoid unnecessary JOINs when simpler EXISTS queries suffice.