HardSQL challenge #329JOINGROUP BYCALCULATION

Calculate Profit Generated by Each Customer

Return each user with the total profit, price minus cost times quantity, across their orders.

The challenge runs on the SQLabHub practice database (SQLite) and uses the products, orders and order_items tables. You can solve it in the browser, with nothing to install.

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The task

Find users generating highest profit (price - cost).

A correct answer returns 114 rows with 2 columns: user_id, profit.

Tables used in this challenge

products 120 rows

id INTEGER (PK) · name TEXT · category TEXT · price REAL · cost REAL · stock INTEGER · sku TEXT · status TEXT · rating REAL · review_count INTEGER

First 3 rows of the products table
idnamecategorypricecoststockskustatusratingreview_count
1Play Kitchen DeluxeToys1786.38589.51257SKU-000001active457
2Micro-Cut Paper Shredder 8-SheetOffice Supplies907.93281.46131SKU-000002active2.5447
34K Dual Dash CameraAutomotive568.89204.8176SKU-000003active4.129

orders 200 rows

id INTEGER (PK) · user_id INTEGER · total REAL · discount REAL · item_count INTEGER · status TEXT · order_date TEXT · shipped_date TEXT · shipping_city TEXT · shipping_country TEXT

First 3 rows of the orders table
iduser_idtotaldiscountitem_countstatusorder_dateshipped_dateshipping_cityshipping_country
1432638.110.253cancelled2024-04-24NULLLos AngelesUSA
2829470.195delivered2024-03-152023-04-09ChicagoUSA
3136683.240.082delivered2023-02-152023-07-10Los AngelesUSA

order_items 496 rows

id INTEGER (PK) · order_id INTEGER · product_id INTEGER · quantity INTEGER · unit_price REAL · subtotal REAL

First 3 rows of the order_items table
idorder_idproduct_idquantityunit_pricesubtotal
11765192.21961.05
21823107.1321.3
316515.395.39

The first rows of each table are shown. See how all the tables relate on the database schema page.

Expected output

Expected output of the challenge
user_idprofit
7731273.379999999997
428290.22
13725442.379999999997
3724714.760000000002
12422874.57

First 5 of 114 rows. This is the output of the model solution on the practice database.

What you practise

  • Summarising many rows into one value with COUNT, SUM, AVG, MIN or MAX
  • Grouping rows with GROUP BY to get one result row per group
  • Combining rows from two tables on a matching key with an inner join
  • Sorting the result with ORDER BY

Hint and approach

Try the challenge first. Open these only when you need them.

Show the hint

JOIN products

Show the step-by-step approach

How the model solution is built, in the order the database evaluates it:

  1. Read from the orders table.
  2. Join order_items on orders.id = order_items.order_id.
  3. Join products on order_items.product_id = products.id.
  4. Group the rows by orders.user_id, so the result has one row for each value.
  5. For each group, compute SUM((products.price - products.cost) * order_items.quantity).
  6. Sort the result by profit descending.

Model solution

Checking your session…

What to watch for

The margin comes from products, the quantity from order_items and the customer from orders, so all three tables are needed. The calculation uses the current list price; a stricter version would use the unit_price recorded on the order line.

  • INNER JOIN. An inner join drops rows that have no partner on the other side, and it repeats a row once for every match, which inflates SUM and COUNT.
  • GROUP BY. Every selected column must either be in GROUP BY or sit inside an aggregate. SQLite accepts other columns and returns an arbitrary value for them; most databases raise an error.
  • ORDER BY. Without ORDER BY the row order is not guaranteed, even if it looks stable. Rows that tie on the sort columns can come back in any order.
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Same technique, other tables

More practice on this pattern with different columns:

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