HardSQL challenge #238WINDOW FUNCTIONAGGREGATIONANALYTICS

Subtract the Overall Average from Each Salary with AVG() OVER ()

Return every employee with the difference between their salary and the average salary.

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.

Solve This SQL Challenge

The task

Find difference between employee salary and avg.

A correct answer returns 120 rows with 13 columns: id, first_name, last_name, email, department, department_id, title, salary, hire_date, manager_id, city, status, diff.

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
id	first_name	last_name	email	department	department_id	title	salary	hire_date	manager_id	city	status	diff
1	William	Lewis	wlewis18@corp.io	Customer Success	7	Senior CS Manager	61774	2022-07-24	NULL	San Diego	active	-62596.84166666666
2	John	Miller	jmiller51@corp.io	Legal	8	Paralegal	191694	2021-08-09	NULL	San Diego	active	67323.15833333334
3	Daniel	Rivera	drivera50@corp.io	Engineering	1	VP Engineering	135067	2016-06-06	NULL	Houston	active	10696.15833333334
4	Melissa	Smith	msmith33@corp.io	Engineering	1	VP Engineering	51465	2022-04-13	NULL	Fort Worth	active	-72905.84166666666
5	Jason	Mitchell	jmitchell78@corp.io	Design	10	Product Designer	156865	2019-10-17	NULL	Denver	active	32494.15833333334

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

Attaching a total or average to every row with an aggregate OVER ()
Deriving a new column from an expression and naming it with an alias

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:

Read from the employees table.
Compute AVG(salary) with an empty OVER (), so the figure for the whole result is repeated on every row.
Combine those values into diff: salary - AVG(salary) OVER ().

Model solution

Checking your session…

What to watch for

AVG(salary) OVER () places the overall average on every row, so it can be subtracted directly. A plain AVG(salary) in the SELECT list would collapse the result to one row.

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.
Calculated columns. Dividing one integer by another truncates the result in SQLite. Multiply one side by 1.0 to get decimals.