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N005-H1 Tier 1 · Foundations · hard ecommerce · Brightlane

Return the ID of every such order

Part of NULL Semantics and IS NULL in SQL

The problem

Brightlane's data governance team is auditing for orphaned orders — orders not linked to any customer account — to flag them for remediation.

Write a query to return the ID of every such order.

Assumptions:

  • The orders table contains every order Brightlane has processed.
  • The customer_id column normally links each order to the customer who placed it.
  • An orphaned order has customer_id set to NULL. Brightlane's referential integrity is currently strong, so it's possible no orders meet this condition; the query should return zero rows in that case.

Output:

  • One row per orphaned order, with the column id. The result set may be empty.
Schema · ecommerce 5 tables
categories
id integer
name text
parent_id? integer
products
id integer
name text
category_id integer
price numeric
stock_qty integer
attributes? jsonb
order_items
id integer
order_id integer
product_id integer
quantity integer
unit_price numeric
customers
id integer
name text
email text
city? text
country text
created_at timestamptz
is_active boolean
orders
id integer
customer_id integer
ordered_at timestamptz
status text
total_amount numeric
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Solution query
SELECT
  id
FROM
  orders
WHERE
  customer_id IS NULL

The shape

An audit query can legitimately return zero rows. That's the point of running it. The data governance team is asking "are there any orphaned orders?" and the answer Brightlane wants is no. The query asks the question correctly; the empty result set is the answer.

Clause by clause

  • SELECT id returns just the order identifier. The governance team only needs to know which orders are orphaned; if any come back, the next step is a remediation lookup that pulls the rest. The minimal projection keeps the audit output focused.
  • FROM orders reads every order Brightlane has processed. The audit needs the full table; sampling would defeat the purpose.
  • WHERE customer_id IS NULL keeps only rows whose customer link is absent. IS NULL returns true for orphans and false for every order linked to a customer. Brightlane's referential integrity is currently strong, so this set happens to be empty today. Returning the IDs directly — rather than just signalling whether any exist — is what makes the result double as both a sanity check (zero rows = healthy) and a worklist (one or more rows = the IDs to remediate).

The trap

The trap on a zero-row result is reading it as a query bug instead of a real answer. "The query returned nothing — it must be broken." It isn't. The query asked a precise question and the answer is precisely none.

The failure mode worth guarding against is the opposite: a query that should find orphans but uses customer_id = NULL instead of IS NULL. That query also returns zero rows, but for the wrong reason — = against NULL evaluates to unknown, so the filter passes nothing whether orphans exist or not. The audit then reports "no orphans found" while orphans pile up unflagged. The way to tell the two cases apart is to read the operator: IS NULL is the correct test; = NULL is the bug that hides the answer.

You practiced a query that may legitimately return zero rows. An empty result set is a real answer — the absence of data is the data, in audit and quality-check workflows.

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