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N053-H3 Tier 4 · Advanced · hard ecommerce · Brightlane

Return the ID, name, and page count for every book product with more than `400` pages

Part of JSONB Field Extraction in SQL

The problem

Brightlane's book recommendation engine surfaces long-form titles for customers who prefer comprehensive texts.

Write a query to return the ID, name, and page count for every book product with more than 400 pages.

Assumptions:

  • The products table has one row per product with an id, a name, and an attributes JSONB column.
  • A book product has a 'pages' key in attributes whose value is a JSONB number.
  • The page-count output column carries the value extracted from attributes and represented as an integer. Only products whose page count is strictly greater than 400 should appear.

Output:

  • One row per qualifying product, with columns id, name, and page_count.
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,
  (attributes ->> 'pages')::INTEGER AS page_count
FROM
  products
WHERE
  (attributes ->> 'pages')::INTEGER > 400

The shape

The recommendation engine needs page counts as integers, and the page values live inside the JSONB attributes document. So extract with ->> to get text, cast with ::integer to make the value numeric, and apply the same extract-and-cast in both the SELECT (for the output column) and the WHERE (for the threshold comparison). Without the cast in the WHERE, the comparison would be lexical and the > 400 filter would behave incorrectly.

Clause by clause

  • SELECT id, name, (attributes ->> 'pages')::integer AS page_count returns the product's ID and name, then extracts the value at the 'pages' key as text and casts it to an integer for the output column. The parentheses around the extraction are required — without them, :: would try to bind to the JSONB literal 'pages' instead of the whole attributes ->> 'pages' expression.
  • FROM products reads the product catalog.
  • WHERE (attributes ->> 'pages')::integer > 400 runs the same extract-and-cast per row and keeps the row only when the integer page count is strictly greater than 400. Products with no 'pages' key extract to NULL, the cast of NULL is NULL, and NULL > 400 is unknown, so non-book products are dropped automatically without needing an explicit existence check.

The trap

Skip the ::integer cast in the WHERE and the comparison becomes a string comparison: '500' > '400' is true (as text and as integers, both), but '99' > '400' is also true as text (because '9' sorts after '4'), even though 99 is clearly less than 400 as a number. The bug is silent: the query runs, returns rows, and the result looks plausible until someone notices a 99-page book in the long-form list. The fix is the cast, and it has to go in both places — the SELECT cast is for display, the WHERE cast is for correctness of the filter. Forgetting either one produces wrong answers without an error.

You practiced (attributes ->> 'pages')::integer in WHERE — chained extraction-and-cast inside a predicate; without the cast the comparison would be lexical (text) rather than numeric.

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