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N054-M2 Tier 4 · Advanced · medium ecommerce · Brightlane

Return a JSON object for every product whose `'format'` attribute is `'Paperback'`. The object must have keys `'id'`, `'name'`, and `'pages'`, where `'pages'` is the value extracted from the product's `attributes` under the `'pages'` key

Part of JSONB Aggregation (jsonb_agg, json_build_object) in SQL

The problem

Brightlane's book catalog export pipeline needs a structured JSON document per paperback product that combines standard product columns with a JSONB attribute.

Write a query to return a JSON object for every product whose 'format' attribute is 'Paperback'. The object must have keys 'id', 'name', and 'pages', where 'pages' is the value extracted from the product's attributes under the 'pages' key.

Assumptions:

  • The products table has one row per product with an id, a name, and an attributes JSONB column.
  • A paperback product has its 'format' key set to 'Paperback' and a 'pages' key in attributes.
  • Only paperback products should appear. Each output row carries one JSON document combining the product's id, name, and the text-extracted 'pages' value.

Output:

  • One row per paperback product, with one column, book_spec, containing the JSON document.
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
  JSON_BUILD_OBJECT('id', id, 'name', name, 'pages', attributes ->> 'pages') AS book_spec
FROM
  products
WHERE
  attributes ->> 'format' = 'Paperback'

The shape

json_build_object accepts any expression as a value, including the ->> extraction from a JSONB column. Mixing the relational columns id and name with the JSONB-extracted attributes->>'pages' inside one call produces a single output document per row, blending data from both sources.

Clause by clause

  • SELECT json_build_object('id', id, 'name', name, 'pages', attributes->>'pages') AS book_spec builds the JSON document for each surviving row. The first two values come straight from the column, so id lands as a JSON number and name as a JSON string. The third value comes from attributes->>'pages', which extracts the 'pages' key from the JSONB column as text — that is why "pages": "431" appears in the result with the value quoted as a JSON string.
  • FROM products reads the product records.
  • WHERE attributes->>'format' = 'Paperback' restricts the input to products whose 'format' attribute is the text 'Paperback'. Only four rows survive the filter — Writing Clean Code, The Efficient Developer, Relational Databases in Practice, and Improving Existing Code — and the function runs once per surviving row.

Why this and not ->

attributes->'pages' returns the JSONB value (which would carry the source type — quoted string if stored as a JSONB string, number if stored as a JSONB number). attributes->>'pages' returns text. The prompt does not require the page count as a number, and the reference result shows "pages": "431" as a string, so the text-extracting ->> is the right choice. Forcing a numeric type would be a separate cast and a different problem.

You practiced mixing relational columns and JSONB-extracted values inside json_build_object — the function accepts any expression as a value, including ->>-extracted strings.

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