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N016-M4 Tier 2 · Core SQL · medium ecommerce · Brightlane

- For orders with `total_amount > $800`, the adjusted total is the original `total_amount` scaled by `0.9` (a 10% reduction). - For all other orders, the adjusted total equals the original `total_amount` (unchanged)

Part of CASE WHEN Expressions in SQL

The problem

Brightlane's pricing team is building a bulk-discount model and needs to simulate adjusted order totals.

Write a query to return each order's ID and adjusted_total:

  • For orders with total_amount > $800, the adjusted total is the original total_amount scaled by 0.9 (a 10% reduction).
  • For all other orders, the adjusted total equals the original total_amount (unchanged).

Assumptions:

  • The orders table contains every order Brightlane has processed.
  • An order priced exactly at $800 is unchanged (threshold is strictly greater-than).

Output:

  • One row per order, with columns id and adjusted_total.
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,
  CASE
    WHEN total_amount > 800 THEN total_amount * 0.9
    ELSE total_amount
  END AS adjusted_total
FROM
  orders

The shape

The THEN branches return numbers, not labels — one branch computes the discounted total, the other returns the original total unchanged. CASE works the same way for numeric outputs as it does for string outputs: each branch just needs to return a value of a compatible type.

Clause by clause

  • SELECT id carries the order ID through so the adjusted total has an identifier next to it.
  • CASE WHEN total_amount > 800 THEN total_amount * 0.9 ELSE total_amount END AS adjusted_total is the derived column. The WHEN tests each order's total_amount against the $800 threshold (strictly greater-than, so an order at exactly $800 falls through to ELSE and is unchanged — matching the prompt directly).
  • THEN total_amount * 0.9 is the discounted branch. The expression references the same column the condition just tested — CASE lets each branch read any column from the current row. Multiplying by 0.9 is the inline form of a 10% reduction (same logic as the parentheses problem in N001: * 0.9 is one operation; total_amount - (total_amount * 0.1) is two).
  • ELSE total_amount returns the original value unchanged. This is the load-bearing detail — it makes the column complete for every order rather than NULL for orders below the threshold.
  • END AS adjusted_total closes the expression and labels the column.
  • FROM orders is the source set: every order, with no filtering.

Why this and not omit the ELSE

Dropping ELSE total_amount would leave orders at or below $800 with NULL in adjusted_total. The pricing model would then have to handle missing values throughout — checking for NULL everywhere downstream, deciding whether to treat them as zero or as the original total. Putting ELSE total_amount in the CASE makes the unchanged path explicit at the source. The branch returns a real number rather than the absence of one, and every order's adjusted total reads as a single coherent column.

You practiced using CASE to produce a derived numeric value rather than a label string. The recurring shape: each branch returns whatever expression you need — strings, numbers, calculations — as long as all branches resolve to a compatible type.

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