Imagine that you are asked to identify the number of days that exist between the first invoice and last invoice for each month and complete the following:
1. Construct a query that will show the number of days that exist between the first invoice and last invoice, for each month, for each employee, using the DATEDIFF function. Be sure to provide the SQL script that will carry out this function.
2. Construct a query to show the expected payment date if invoices are due within 30 days of transaction.
3. Construct a query that will show distinct area codes of the customers.

Create a plan of the necessary activities that would be required to implement a valid database design process by including the following:
1. Steps in the conceptual design stage
2. Steps in DBMS selection stage
3. Steps in logical design stage
4. Steps in physical design stage
5. Task details of each activity within each stage