[StrataScratch] Salaries Differences [MySQL]

https://platform.stratascratch.com/coding/10308-salaries-differences?code_type=3 

StrataScratch

SELECT ABS(MAX(db_employee.salary) - (
    SELECT MAX(db_employee.salary)
    FROM db_employee
    GROUP BY department_id
    HAVING department_id = 4
))
FROM db_employee
GROUP BY department_id
HAVING department_id = 1