HardSQL challenge #214SELF JOINDATEROLLING WINDOW

Count Orders in a Rolling 7-Day Window

For each order date, return the number of orders placed in the 7 days ending on that date.

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

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The task

For each order date, count the orders placed in the 7 days ending on that date.

A correct answer returns 182 rows with 2 columns: order_date, orders_7d.

Tables used in this challenge

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

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
order_dateorders_7d
2022-01-061
2022-01-102
2022-01-152
2022-01-163
2022-01-173

First 5 of 182 rows. This is the output of the model solution on the practice database.

What you practise

  • Filtering a range of values with BETWEEN
  • Removing duplicate rows with DISTINCT
  • 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
  • Querying the result of another query with a subquery in FROM
  • Working with dates in SQLite: strftime, julianday and DATE
  • Sorting the result with ORDER BY

Hint and approach

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

Show the hint

A ROWS frame counts rows, not days. Self join the distinct dates to orders on a 7-day date range.

Show the step-by-step approach

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

  1. Build an inner query first (a derived table):
    1. Read from the orders table.
    2. Return only the column order_date.
    3. DISTINCT removes duplicate rows, so each value appears once.
  2. Join orders on the condition where orders.order_date is between DATE(d.order_date, '-6 day') and d.order_date, both ends included.
  3. Group the rows by d.order_date, so the result has one row for each value.
  4. For each group, compute COUNT(*).
  5. Sort the result by d.order_date.

Model solution

Checking your session…

What to watch for

A frame such as ROWS BETWEEN 6 PRECEDING counts rows, so it only means seven days when every day has exactly one row. Here the window is a date range: each distinct date is joined to every order from the six days before it up to the date itself.

  • Derived table. Columns computed in the inner query are only visible outside under their alias, so every calculated column there needs a name.
  • 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.
Solve This SQL Challenge

Same technique, other tables

More practice on this pattern with different columns:

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