why didn't we use hire date from employees instead of from_date from dept_emp?

Hi Mai!
Thanks for reaching out.

The hire_date column indicates the date that the employee has started working. The from_date and to_date columns indicate the start date and the end date for the current work that the employee has worked. Let's check for emp_no=10001. If we run the following query:

SELECT 
    *
FROM
    employees
WHERE
    emp_no = 10001;

we see Georgi Facello has started working at 1986-06-26. This is his career start for the company. But in this company he has worked at different positions during different periods. If we run the following query:

SELECT 
    *
FROM
    salaries
WHERE
    emp_no = 10001;

we see all different periods of work with all salaries and the columns from_date and to_date for all his previous jobs. If he is currently working at particular position the date is set to 9999-01-01.

Hope this helps.
Best,
Tsvetelin