This content originally appeared on DEV Community and was authored by Dror Atariah
I am sure you saw it a zillion times, but it can still hit you if you're being sloppy.
The uniqueness of your left table is not guaranteed to hold after a left join.
Assume that your left table is:
| customer_id | customer_name |
|---|---|
| 1 | Alice |
| 2 | Bob |
| 3 | Charlie |
and your right table is:
| product_id | customer_id |
|---|---|
| a | 1 |
| b | 1 |
| b | 2 |
| c | 2 |
| c | 3 |
Then, the result of the join would be:
| customer_id | customer_name | product_id |
|---|---|---|
| 1 | Alice | a |
| 1 | Alice | b |
| 2 | Bob | b |
| 2 | Bob | c |
| 3 | Charlie | c |
This is a very straightforward example, so you might not be surprised.
But, rest assured, in more complex setups you can easily miss that and end up with duplications in your result set.
Data quality
Data quality issues can mean that your understanding of the data is misaligned with the actual nature of the data you use.
Namely, you read a right table where you expect it to have unique identifiers, but in reality it has duplications.
These duplications will end up in the result of the join!
So the lesson learned is: ALWAYS CHECK PRE-CONDITIONS!
This content originally appeared on DEV Community and was authored by Dror Atariah
Dror Atariah | Sciencx (2025-07-22T21:10:00+00:00) Left Join Gotcha: Rows Duplication. Retrieved from https://www.scien.cx/2025/07/22/left-join-gotcha-rows-duplication/
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