HardSQL challenge #204WINDOW FUNCTIONMOVING AVERAGEANALYTICS

Calculate a Moving Average with ROWS BETWEEN

Return each order date with the average of that order and the two before it.

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

Calculate 3-day moving average of order totals.

A correct answer returns 200 rows with 2 columns: order_date, moving_avg.

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_datemoving_avg
2022-01-061843.49
2022-01-102881.965
2022-01-152305.3533333333335
2022-01-162821.41
2022-01-172902.5200000000004

First 5 of 200 rows. This is the output of the model solution on the practice database. The task does not fix a sort order, so your rows may come back in a different order.

What you practise

  • Limiting a window to nearby rows with ROWS BETWEEN

Hint and approach

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

Show the hint

ROWS BETWEEN

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. Compute AVG(total) as a window function over the current row and the 2 rows before it, ordered by order_date.

Model solution

Checking your session…

What to watch for

ROWS BETWEEN 2 PRECEDING AND CURRENT ROW counts rows, not calendar days: it averages three consecutive orders, which may fall on one day or several weeks apart. A true 3-day average needs one row per day first. The first two rows average over fewer than three values.

  • Window frames. A ROWS frame counts rows, not time. "6 PRECEDING" is six rows back, which is six days only when there is exactly one row per day.
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Same technique, other tables

More practice on this pattern with different columns:

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