HardSQL challenge #256WINDOW FUNCTIONRUNNING AVERAGEANALYTICS

Calculate a Running Average Salary by Hire Date

Return each hire date with the average salary of everyone hired up to that date.

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

Calculate running average salary by hire date.

A correct answer returns 120 rows with 2 columns: hire_date, running_avg.

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
idfirst_namelast_nameemaildepartmentdepartment_idtitlesalaryhire_datemanager_idcitystatus
1WilliamLewiswlewis18@corp.ioCustomer Success7Senior CS Manager617742022-07-24NULLSan Diegoactive
2JohnMillerjmiller51@corp.ioLegal8Paralegal1916942021-08-09NULLSan Diegoactive
3DanielRiveradrivera50@corp.ioEngineering1VP Engineering1350672016-06-06NULLHoustonactive

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
hire_daterunning_avg
2015-01-14204449
2015-02-16185368.5
2015-02-26174297
2015-03-19181589.5
2015-04-24161540.6

First 5 of 120 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

  • Accumulating a value row by row with an ordered window

Hint and approach

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

Show the hint

AVG OVER

Show the step-by-step approach

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

  1. Read from the employees table.
  2. Compute AVG(salary) as a running window function: it covers the rows from the start up to the current one, ordered by hire_date.

Model solution

Checking your session…

What to watch for

AVG() with ORDER BY inside OVER averages from the first row to the current one, so the window grows with every row. A moving average is different: it uses a fixed-size frame.

  • Running totals. With ORDER BY and no frame, rows that tie on the sort column are added together in one step. Use ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW for a strict row-by-row total.
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Same technique, other tables

More practice on this pattern with different columns:

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