I need to calculate the cost for each project for each month, the project consists of tasks. I have an employee table, tasks table and project table. But I don't know how to calculate the cost for a project each month, so far I have this:

```
SELECT 
    P.PROJECT_NAME, 
    SUM(T.HOURS_WORKED * E.HOURLY_RATE) COST
FROM 
    PROJECT P,
    TASKS T,
    EMPLOYEE E
WHERE 
    E.EMPLOYEE_ID = T.EMPLOYEE_ID
    AND P.PROJECT_ID = T.PROJECT_ID
GROUP BY P.PROJECT_NAME;

```

but that doesn't work out how much is charged for a project each month,it just works out the overall cost by calculating the hours worked by the employees by the employee hourly rate. In the task table I do have a date\_worked column which displays the day,month and year but I don't know if that needs to be used or not.