hard20 pts · first answer countssqlnull-handling

NOT IN returns nothing

This query returns zero rows, even though many customers have no refunds. refunds.customer_id contains some NULL values. Why?

SELECT id FROM customers
WHERE id NOT IN (SELECT customer_id FROM refunds)

A.NOT IN cannot be used with subqueriesB.The subquery runs once per row and times outC.customers.id must also be NULL for the match to workD.If the list contains a NULL, "id NOT IN (...)" becomes UNKNOWN for every row, so no rows pass

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