Understand why SUM(SUM(x)) OVER () is valid and what it computes over grouped rows.
Consider:
SELECT department,
SUM(salary) AS dept_total,
SUM(salary) / SUM(SUM(salary)) OVER () * 100 AS pct_of_company
FROM employees
GROUP BY department;
What does the expression SUM(SUM(salary)) OVER () evaluate to for each row?