HardSQL challenge #243WINDOW FUNCTIONSTATISTICSAGGREGATION

Median Order Value

Find median value of order totals.

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

Find median value of order totals.

A correct answer returns 1 row with one column: AVG(total).

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
AVG(total)
2499.62

The complete result: 1 row. This is the output of the model solution on the practice database.

What you practise

  • Filtering rows with a WHERE condition
  • Summarising many rows into one value with COUNT, SUM, AVG, MIN or MAX
  • Querying the result of another query with a subquery in FROM
  • Numbering rows with ROW_NUMBER() to pick one or a few per group
  • Attaching a total or average to every row with an aggregate OVER ()

Hint and approach

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

Show the hint

ROW_NUMBER + COUNT

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. Number the rows with ROW_NUMBER(), ordered by total.
    3. Compute COUNT(*) with an empty OVER (), so the figure for the whole result is repeated on every row.
  2. Filter in WHERE: keep the rows where rn is one of (cnt+1)/2 or (cnt+2)/2. The filter sits in the outer query because a window function cannot be tested in the WHERE clause of the query that computes it.
  3. Aggregate over all the rows that are left: compute AVG(total). With no GROUP BY, the result is a single row.

Model solution

Checking your session…

What to watch for

  • ROW_NUMBER(). ROW_NUMBER() breaks ties arbitrarily. When the ORDER BY inside OVER does not identify one row, the row that gets number 1 can change between runs.
  • Window aggregates. A window aggregate keeps every row, while a plain aggregate collapses them. Mixing the two in one SELECT without GROUP BY is an error.
  • Derived table. Columns computed in the inner query are only visible outside under their alias, so every calculated column there needs a name.
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