Conditional COUNT of field in related table
I am creating a database for my firm that has two tables: Customer (Parent) and Invoice (Child). The aim of the database is to carry out various analysis. I would like to COUNT a specific field in the Invoice table that has a "Paid" or "Unpaid" answer to show management how many total 'Unpaid' invoices exist. This is equivalent to Excel COUNT(IF)
I have tried using 'If' statements but to no avail. As a newbie I'm not sure where the correct approach would be using CASE and a variable or LOOP function.
Thanks in advance for any advice.