Join fan-out

Also called: row multiplication

When a join to a one-to-many table repeats each row once per match, so aggregates of the “one” side are counted several times.

Formularows after join = Σ matches per row, not one per row

Try it · 30 seconds · from Atterna’s practice

orders.id	amount	items in the order
1	€50	3
2	€30	1

SELECT SUM(o.amount)
FROM orders o
JOIN items i ON i.order_id = o.id;

What does this query return?

Preparing the practice…

How we work it through

The query we run on Meridian, Atterna’s practice marketplace, and what it returns.

SELECT COUNT(*), SUM(o.items_total_eur)
FROM meridian.orders o
JOIN meridian.order_items i ON i.order_id = o.order_id
WHERE o.status = 'paid';

Result16,192 rows and €1,239,938 — 2.2× the real GMV of €552,645

SUM after the join: no error, just a total that is too high.

ExampleMeridian: 9,071 paid orders become 16,192 rows after joining order items; summed GMV rises from €552,645 to €1,239,938.

An AI assistant

“Total revenue from paid orders is €1,239,938.”

What it didn’t know: It joined order items to add product names and did not notice each order now repeats.
Ask back: “Did the join repeat orders? Show the row count before and after it.”