<<customers>>
id | first_name | last_name
1  | Reed       | Richards
2  | Johnny     | Storm
3  | Peter      | Parker

<<purchases>>
id | cid | date
1  | 1   | 2017-01-09
2  | 2   | 2017-01-09
3  | 2   | 2017-01-09
4  | 3   | 2017-01-09

SELECT
    COUNT(c.id) as "Total Customers",
    COUNT(p.id) as "Total Sales",
    COUNT(c.id)/COUNT(p.id) as "Sales per customer"
FROM test_customers c
    LEFT OUTER JOIN test_purchases p ON c.id = p.cid

4 | 4 | 1

3 | 4 | 1.3333333...

SELECT
    COUNT(distinct c.id) as "Total Customers",
    COUNT(distinct p.id) as "Total Sales",
    COUNT(distinct c.id) * 1.00 / COUNT(distinct p.id) as "Sales per customer"
FROM test_customers c
    LEFT OUTER JOIN test_purchases p ON c.id = p.cid