Why should you care?

Pick the latest order per customer for a snapshot export — row number 1 after ORDER BY placed_at DESC.

See it live — copy this example

Run in MySQL Workbench or the mysql CLI.

SELECT * FROM (
  SELECT o.*,
         ROW_NUMBER() OVER (
           PARTITION BY o.customer_id
           ORDER BY o.placed_at DESC
         ) AS rn
  FROM orders o
) t
WHERE t.rn = 1;

What happened?

• Inner query numbers each customer’s orders newest-first.
• Outer filter rn=1 keeps latest order only — classic dedupe pattern.

Practice next

1. Insert multiple orders per customer with different dates.
2. Run inner query only — inspect rn values.
3. Filter rn=1; verify one row per customer.
4. Change to rn <= 3 for last three orders.
5. PARTITION BY city via JOIN to customers.

Remember

ROW_NUMBER is unique per partition. ORDER BY inside OVER controls ranking. Filter in outer query for top-N per group.

DataFlow last order export

CRM sync sends only latest order per user nightly.

Outcome: Downstream warehouse avoids duplicate customer rows.