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N008-H2 Tier 1 · Foundations · hard ecommerce · Brightlane

Return the ID, name, and price for every product in that batch

Part of LIMIT and OFFSET in SQL

The problem

Brightlane's export tool is requesting the batch that would cover positions 71 through 80 of the catalogue, using the same page size of 10 and alphabetical sort.

Write a query to return the ID, name, and price for every product in that batch.

Assumptions:

  • The products table contains exactly 63 products in Brightlane's catalogue.
  • The batch starts at position 71 — OFFSET 70, then LIMIT 10.
  • Position 71 is past the end of the catalogue (63 < 71), so the result set will contain zero rows. PostgreSQL returns an empty result, not an error.

Output:

  • One row per product in the batch, with columns id, name, and price, sorted by name ascending.
  • The result set will 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,
  name,
  price
FROM
  products
ORDER BY
  name
LIMIT
  10
OFFSET
  70

The shape

The export tool asks for positions 71 through 80, but the catalogue only has 63 products. OFFSET 70 lands past the end of the result set, so the query returns zero rows without raising an error.

Clause by clause

  • SELECT id, name, price picks the three export columns. The shape of the SELECT list is unchanged from the previous batch query — only the offset moves between requests.
  • FROM products reads the full catalogue. The query plan still computes the entire sorted result before the window is applied.
  • ORDER BY name sorts alphabetically. The sort still runs even though no row will survive the window, because PostgreSQL can't know the offset is out of bounds until after it has the sorted set in hand.
  • LIMIT 10 OFFSET 70 requests up to 10 rows after skipping the first 70. The sorted result has 63 rows; PostgreSQL skips through all of them, finds nothing left, and returns the empty result set. No error, no warning, no padding.

Why this and not a row-count check first

The export tool can ask the question and read the answer instead of asking how big the catalogue is before each request. An empty result is a meaningful answer in itself: "there is no batch at this position." A defensive count query would double the round-trips and still wouldn't help — the count could be stale by the time the second query runs.

This is also why the same query can return rows tomorrow without any code change. As more products are added, the catalogue grows past 70 rows, and the same query starts returning data. The query stays static; the data moves underneath it.

The trap

An empty result set is not an error. A learner who treats zero rows as a failure case will write retry logic or assume the query is wrong, when in fact it has done exactly what it was asked to do — return whatever rows live inside the window. The executable rule: an out-of-bounds OFFSET returns zero rows, the same way an in-bounds OFFSET past the end of the data returns a partial batch. Both are valid responses to a valid query. The signal isn't "something broke," it's "nothing falls inside this window right now."

You practiced requesting an OFFSET that lands past the end of the result set. The recurring lesson: an out-of-bounds OFFSET returns zero rows, never an error — useful to know because the same query may return rows tomorrow as more data arrives.

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