HardSQL challenge #219WINDOW FUNCTIONSTATISTICSAGGREGATION

Calculate the Median Salary Without a MEDIAN Function

Return the median employee salary using ROW_NUMBER() and COUNT() OVER.

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

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The task

Find median salary of employees.

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

Tables used in this challenge

`employees` 120 rows

id INTEGER (PK) · first_name TEXT · last_name TEXT · email TEXT · department TEXT · department_id INTEGER · title TEXT · salary REAL · hire_date TEXT · manager_id INTEGER · city TEXT · status TEXT

First 3 rows of the employees table
id	first_name	last_name	email	department	department_id	title	salary	hire_date	manager_id	city	status
1	William	Lewis	wlewis18@corp.io	Customer Success	7	Senior CS Manager	61774	2022-07-24	NULL	San Diego	active
2	John	Miller	jmiller51@corp.io	Legal	8	Paralegal	191694	2021-08-09	NULL	San Diego	active
3	Daniel	Rivera	drivera50@corp.io	Engineering	1	VP Engineering	135067	2016-06-06	NULL	Houston	active

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(salary)
121791

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:

Build an inner query first (a derived table):
1. Read from the employees table.
2. Number the rows with ROW_NUMBER(), ordered by salary.
3. Compute COUNT(*) with an empty OVER (), so the figure for the whole result is repeated on every row.
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.
Aggregate over all the rows that are left: compute AVG(salary). With no GROUP BY, the result is a single row.

Model solution

Checking your session…

What to watch for

Number the salaries in order and count them. Integer division makes (cnt+1)/2 and (cnt+2)/2 the same middle position when the count is odd and the two middle positions when it is even; averaging those rows gives the median either way.

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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