Use a UNION to create a list of all the employee's and customer's first names and last names ordered by the last name in descending order.

Sure, I can help with that. However, I need to know the structure of your tables to provide an accurate SQL query. Assuming you have two tables, employees and customers, and both of them have first_name and last_name columns, the SQL query would look like this:

SELECT first_name, last_name
FROM employees
UNION
SELECT first_name, last_name
FROM customers
ORDER BY last_name DESC;

Here's a step-by-step breakdown of the query:

1. SELECT first_name, last_name FROM employees: This part of the query selects the first and last names of all employees.

2. UNION: This keyword combines the results of two or more SELECT statements without returning any duplicate rows.

3. SELECT first_name, last_name FROM customers: This part of the query selects the first and last names of all customers.

4. ORDER BY last_name DESC;: This part of the query sorts the combined list of employees and customers by last name in descending order.

Please replace employees and customers with your actual table names, and first_name and last_name with your actual column names if they are different.