Auto Populate a field

[Please_help_me]

I have a table with list of department and dept_id (Table private) and also another table (table complete)

when i input in the (complete table), they fill in multiple data, include the department and dept_id and i use drop down from (table private).
is there anyway the dept_id field to get automatically populated base on the selection i made on department?

Please help.
Thank you