Join orders with the customer master

Question

Orders carry only a `customer_id`. To report by customer tier you need the customer master joined in — and orders from unknown customers must not be silently dropped.

Write a function `enrich(orders, customers)` that left-joins `customers` onto `orders` on `customer_id`, and fills a missing `tier`

Accepted Answer

Step 1: `orders.merge(customers, on="customer_id", how="left")` keeps every order row. An inner join — the pandas default — would quietly delete the order from customer 999. Step 2: Unmatched rows get `NaN` in the columns coming from the right frame, so `tier` is null for order 5. Step 3: `fillna("Unknown")` replaces those nulls with an explicit label, which is far easier to spot on a dashboard than a blank. Step 4: Customer 104 has no orders and correctly does not appear — a left join only keeps right-side rows that match.

Join orders with the customer master

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