Percent of Total

"What share of the payroll does each person take?" You need each row's value and the grand total at the same time. A window with an empty OVER () puts the total on every row.

SELECT first_name, salary,
       ROUND(100.0 * salary / SUM(salary) OVER (), 1) AS pct_of_payroll
FROM employees
ORDER BY salary DESC;

Add PARTITION BY to get a share within each group:

SELECT first_name, department_id, salary,
       ROUND(100.0 * salary / SUM(salary) OVER (PARTITION BY department_id), 1) AS pct_of_dept
FROM employees
ORDER BY department_id, salary DESC;

Why 100.0 and not 100? Salaries are whole numbers, and dividing whole numbers drops the decimals, so Alice's 5.7% would come out as 5. Writing 100.0 makes the math keep its decimals.