Interview › IN vs EXISTS

SQL interview: IN vs EXISTS vs JOIN

A subquery that can return NULL makes NOT IN return no rows. EXISTS does not have that mood.

Prompt. Customers who have at least one shipped order.

Hint: WHERE EXISTS (SELECT 1 FROM Orders o WHERE …)

Your T-SQL

SELECT c.customer_id, c.email
FROM Customers c
WHERE EXISTS (
  SELECT 1 FROM Orders o
  WHERE o.customer_id = c.customer_id
    AND o.status = 'Shipped'
);

A JOIN can duplicate customers if they have two shipped orders. EXISTS will not.

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