Doing a count based on the values in multiple tables

Hello everyone - hope you're well.

I have one table which is a list of all our staff, both past and present, each with a unique ID.

Using the unique ID, this table has a 1 to many relationship with 2 other tables:

• A list of departments they have been a member of with the date they started in the department
• A list of the number of hours they do a week with the date that any change to their hours was made - effectively these hours divided by the standard week of 37.5 hours gives how many FTEs they are.

I also have a separate date table.

So a staff member may both change departments while they are working for us and change their weekly working hours.

I'd like to create a measure to calculate over a given period:

• Average FTEs for a department over a chosen period
• values to enable a line graph to show the average FTEs profile over a chosen period

I'm guessing it's either going to:

• Some sort of joint filtering
• Some way of joining up the two changes tables into one?